I believe they can (depending on the file format, I can't remember what file formats T.Ed supports off the top of my head) but I don't see much purpose in importing "characters" into T.ed because they would be stationary. Boxes, crates etc would be of better use I think. Just remember when importing the objects into T.ed there may be licensing issues if you decide to go commercial with your games/terrains.
As for importing T.ed terrains into FPSC it may technically be possible but I don't think it will work the way you intend it to. As far as I know (I haven't been up to date with FPSC for over a year) FPSC doesn't support terrains so there may be issues with collision. There is nothing to stop you from having a go though.