This kind of situation is difficult There are some recent threads that dealt with various aspects of the problem and you might like to check them out. Off-hand I do not remember the exact names of the threads, but I will try to collect them together .
Basically, you in-house cat may never recover her sweet nature if you introduce another adult cat into your home. You could more easily introduce a new dog.
There are a number of things you can try. In brief:
put the newcat in another room with a litter box and a niche to sleep in (a cat carrier without its door, a blanket on a chair, etc). Feed it there and let it be essentially locked away for a week or so.
Routinely take the blanket the cat sleeps on and, before you wash it, put it where Sam can investigate it for a day or two.
Give Sam a great deal of extra attention.
Also give the newcat a great deal of attention.
Let each cat smell the other on your clothing.
Let the newcat out of the room for short periods of time with you supervising. Do not interrupt acquaintance rituals (hissing, brandishing claws, extremely tense posture or stalking) for the first few seconds. Then talk to both cats. Deliberately reach down and pet each cat a light touch on their heads and pull back, all the time talking soothingly about how good they are and how you love them and that they really don't want to fight (or you can quote Shakespear in an intimate, baby-tone voice!).
If it looks dangerous, pick up the nearest cat and very quickly and firmly put it in the isolation room. It doesn't matter which cat, since the idea is 1) separation, and 2) getting familiar with each other's smell with yours overlaying both.
At some point, you can let the cats out into the rest of the house and offer them some special food in their own dishes. Put the dishes down when the cats are at opposite sides of the room -- move quickly, but smoothly from one cat to the other, so they don't have time to try to jump on the food together. If you are indecisive, they have time to figure out that you are possibly putting down food for one and not the other.
Their tendency will be to gulp the treat and then try to steal the other's. by this time you should have the broom in your hand (having placed in at a midpoint between the two cats before you bring out the food. Use it to gently push whichever cat has finished first and is now advancing on the other. If the broom doesn't work (DON'T HIT with it -- just use it for gentle crowd control), snatch up the advancing cat and put it in the separate room. Repeat this maneuver once or twice every day at about the same clock time, so the cats get used to it as a routine.
This is all a play-it-by-ear process that takes a great deal of patience and regularity.
There are other things -- I will try, as I said, to refer you to the various threads that I have seen, but I am in the midst of a lot of work, and do not have "pleasure" time just now.
At best, the two cats will generally ignore each other except when they feel obliged to attack for some real or imagined invasion of areas they consider their own private places in the house.
At worst, you will end up with two cats who become near psychotic in temperament, and you will have to choose one and try very hard to give the other one away to someone nice. Much depends on your own approach. The main word is Love -- lots of it to both cats. The second most important word is Patience (as are the 3rd, 4th, 5th, ad infinitum).
I hope you are successful. Just don't give up.
I wish you peace and harmony,