Don't do it.
Flats are not good for slapping.
Flats do somewhat limit the versatility of a jazz bass.
I play flats, but I like Precision style basses which matchup well with flatwounds.
If by the sliding sound of nickels, you mean finger noise-it goes with the territory and if you can learn to be more careful, you can limit the noise in between the notes.
I recommend trying other rounds like Sunbeams, Low Riders and stainless steel, but I think they are harder on the fingers.
I use nickel rounds.
There are dozens of very long threads on the subject. Use the search function to find them.
Like these:
http://www.talkbass.com/forum/f16/confessions-flatwound-convert-pt-2-a-685391/