If they aren't collector knives and you want a usable handle finish on the micarta, there are several options, but all will probably ruin most of the value of the knife. With that in mind, if its a custom, you may want to have the maker do the work on it.
With the right finish, micarta can be one of the most "grippy" materials. Unfortunately, a lot of makers polish them so they become slippery. The first option is to start with some 100-150 grit sandpaper and sand the handle. Once done with the coarser grit, go to a 220, 320, and finally finish off with 400 grit. It makes a nice grippy surface that still looks good. If you want even more grip, go back with some coarse shopcloth.
You can also file grooves in the micarta. Use a round or triangular file.
Finally, you can have the scales bead or sand blasted. This will give the grittiest finish of all. You'll need to tape off the blade if you don't want it blasted too.
David