Or you could just copy the configs from the tekkit world to the FTB mods folder. The map should load fine then, assuming you use the same set of mods.
I don't know which version is tekkit at right now (or more exactly which version is your save file for), if you're migrating from an older version (1.3.x) you might run into problems. Also obviously items from mods that are in your tekkit installation and aren't in your FTB one will disappear, unless you add the mods manually.
Good luck, and if you do manage to get it to work, maybe you can post a short guide!I am starting to convert the ids. Wish me luck.
It is going to take a lot longer than I thought.How'd you go with this task? Was it worth converting the whole world?