There's no reason the Klingons couldn't be let in if the right diplomats were there !
Of course, this is assuming that the Klingons WANT to enter the Federation. Many of the Great Houses would see entry into the Federation as an act of betrayal and there would be civil war.
Hmmmmm......hang on.....that sounds like an inkling for a few episodes, if not a series! :)
Cheers
Tas
