To best make an outdoor kitty an indoor kitty, you'll need to provide a lot of toys and such so that she has mental stimulation and physical exercise. If she's tame and not at all feral, the transition should be relatively easy.

Like Jazzcat said, it is definately in her best interest to be spayed. Even if you do bring he in fulltime, she should still be spayed. Once a cat goes into heat, they will be determined to get out. It also helps prolong her life and depletes the risk of certain cancers.