20 feral cats!

So I've started volunteering at my local no kill shelter, mostly in the cat section. One of my duties is to care for about 20 feral cats who are permanent residents at the shelter (they were rescued from being culled by the government), and i want to know if there's any tips on how to befriend them?
There are two ferals (Venus and Jelly) who are reasonably friendly, but the rest are, and always have been, terribly wild. Most are from the same family/situation, but two brothers in another section of the shelter (the non rehomable section) are really really beautiful long haired tabbies, but they hiss at me any time i get within 2 meters of them. Is there any way i can get them to feel comfortable around me?

Like I tell my grandson, MOVE SLOW AND SPEAK QUIETLY! Cats react strongly to noisy or quick movements. Never stare into their eyes, they take this as an act of aggression. Sit quietly and let them get used to your smells and the sight of you. Bring small treats to throw to them, they may not eat them with you there, but will later.   Remember that these beautiful creatures are so scared of all these strange humans and being caged up, it is up to you to convince them they are so much better off than trying to live in the wild. Time is the only thing that truly works, you can't force them to accept you. When cleaning their areas, move slowly and talk calmly to them, about anything, they need to get used to your voice. One day it will happen, each in their own way, you will be accepted and they will allow you into their circle. It is one of the most rewarding things you will ever gain, a treasure. Even if they keep hissing, they eventually can allow you close, as close as anything will be allowed in the world of a feral cat. Bless you and that shelter for helping them, so few do. They don't know how lucky they are! I'll keep you in my prayers, don't get discouraged, one day it will happen! 

"MOVE SLOW AND SPEAK QUIETLY!"

as well as

"Time is the only thing that truly works, you can't force them to accept you."

is the best advice you could get. 

And I am with @Di and Bob  on "Bless you and that shelter for helping them"

Thank you both, i am definitely aware that i can't force them to like me, i just want to make sure I'm not doing anything wrong, that might make them uncomfortable most of them have been at the shelter for several years, and will be there until the end of their lives, but i assure you they all live in big enclosures with lots of shelter, toys, food and litter areas which are cleaned daily
I managed to get slightly close to one of the brothers (Tom and Jerry) today, but i definitely respect them and try never to handle a cat who doesn't want to be handled i just want to make their lives as fun and comfortable as possible, thanks for ur advice!

Provided you are caring and patient there is very little you can do wrong. Expect that at times it will be one step forward and two steps back and don't get discouraged. It seems you are well on your way to making their lives as fun and comfortable as possible.

I've found that not just staring into their eyes, but even simply glancing toward them can spook them. So, I took to wearing a wide-brimmed hat, worn cocked over my face so I can only see them below their eyes (often, just their feet), so I at least know where they are. Otherwise they would quickly run off & disappear. (This was in an open area.)  Doing this, Some of them eventually came up & ate right in front of my feet, with me sitting right there, close enough to touch, hiding behind my wide-brimmed hat.

I also recommend EFT / Emotional Freedom Technique. Many have learned how to apply it themselves, for both family & pets.

One can learn how to neutralize the negative memories & feelings so they stop causing problems in both physical & emotional health, using EFT.

"EFT for Animals"

http://www.efthelps.com/category/eft-for-animals/

There is also music designed to sooth cats in particular, which you may want to try:

"The Music That Is Appealing to Many Cats"

http://healthypets.mercola.com/sites/healthypets/archive/2016/03/17/music-for-cats.aspx

I want to add, not just talking calmly in a friendly low voice.  But also making different friendly noises.  Compare for example, how a friendly courting tom is doing, with lotsa of friendly noises.  Its good cat language.

I had a coworker where she and her husband had marvellous communication with the pack of ferales at their home, mainly talking with them...  It worked even with the fox and the visiting elks...  They could steer whose turn it was to go forward and eat... Or the mama to show where she had her hidden nest with kittens.

Standing on all four, strech yourself forwards or backwards.  Stretching is for cats greeting pals.

Yawning can also work this way, at least, it worked alike a marvel with my residents.  We possibly learned each other this...  As I have noticed other cats not always react positively on this...

The old goodie of slowly blinking is a safe tip.