Fractal Terrains can import existing worlds. In my opinion it's not terribly easy to work with on existing worlds, however. You can do it, but you need great care and patience. FT is geared primarily towards output for CC3, which might mean that you would want to invest in that software as well. What features of FT are driving your interest toward it?

However, it is easy enough to use PS Elements for making better maps. If you're after artistic-type maps then it is quite well suited for that, as is the GIMP (free software). There are numerous tutorials around here for both software packages and I recommend taking a look at them if you haven't before selecting a particular piece of software.