waknstak,
Yes, you can do that. I've done it. It's kind of a pain, but it works if you are patient. It would probably add less weight than doubling the serving, but I've never weighed a string either way, so I'm not sure.
I did it for my daughter's bow. It's a 45# @ 28", but she probably only draws about 26. I made her a 10 strand B50 and padded the area under the serving with four strands to get the nock fit I wanted. It makes a faster string, but I think it is a bit noisier. Haven't made any with FF yet, but my Kanati came with a D97 string. I don't know the strand count though.
Russ