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It should be, since vanilla minecraft is completly existent in FTB.
But you won't have all the stuff necessary for some of the FTB-Mods.
The folder of the respective FTB-Pack should have a save-folder inside (basically the Pack-Folders have a .minecraft-folder like minecraft normally has, but without the dot), just copy your save from your old save-folder to the new one.
Just like he said. Just be aware that no new terrain gen would appear unless you went to new chunks. So stuff like new biomes, ores, and the like. I believe that thaumcraft has a way to add its stuff back in. But I'm not sure on the process.
So, yes. You can have your old digs (country estate, maybe?), and then generate a Mystcraft world as a resource gathering age and transport everything back home. Actually, that sounds like something I need to try!