you CANT use a TSF level in 1.23.
other way around can. (use 1.23 levels in TSF)
its very difficult (but possible) to change a TSF level into an 1.23 level, but most likely you arent gonna get it to work.

(it involves replacing part of the level file (the "header") with teh smae part of an 1.23 file, from a level with exactly the same *filesize* (so not teh same number of tiles.)