this was a prob for me too mate.
What you need to do is work out the portamento settings on each vst and adjust the glide knob so that the notes slide up and down towards the next note.
you can do this on fruity channel settings too.
Not saying ive been doing this myself, but thats what you need to do.
anyone else have a better solution?
yeah you can only use slides with FL instruments and wave files in the sampler
the only way id guess you can do it is by resampling and then loading the sound as wav like fratanize said
or maybe automate the pitch???