Chop all your samples in Soundforge. Set each sample as an audio file in the step sequencer so you can timestretch them easily in the playlist, not to be confused with the piano roll. First find the tempo of them by clicking detect tempo but don't change the project tempo to the sample tempo...
Provided that the version you have is 6.0.8, then you shouldn't have any problems. If so, it'd be the first time I've heard of not being able to use them in FL.