Hey Knn,
Off hand I don't think you can unfortunately.
Though I can imagine situations where it could be handy.
My typical work around is to make a copy of the smart object, hide the original, then rasterize and filter the copy. If I ever need to change the smart object, I can change the original and repeat the copy-and-filter process. It's not exactly elegant, but it works okay in a lot of cases.
As for FF as a smart filter, I think I might have an idea as to why it hasn't been given that functionality.
If FF ran as a smart filter, any change that required re-processing would force the Filter Forge engine to reprocess the image.
That might not be too bad for simple FF filters but for some of the more complex ones it could be a problem if Photoshop regularly became unresponsive while the filter was reapplied.
I'm not sure how smart filters are handled behind the scenes in Photoshop, but if the FF engine had to be launched and closed each time a smart filter needed an update, that could make things even slower.
Just conjecture though.