No, you can't use treat() to do an F-test. I'm not even sure what a fold-change threshold would mean for an F-test. Fold changes are only defined for individual contrasts, not for an F-test, which in effect tests all possible contrasts.
Of course, if you just want a gene to satisfy a treat() test for several different contrasts, then you could just run treat() for each of those contrasts. The decideTests() function provides an easy way to select genes satisfying several contrasts at once.