mak 90 barrel mod

Is it possible to thread the barrel of a not threaded MAK-90. I want to switch site bases and thread for std. barrel nut.

Yea, don't see why not,if they didn't turn off the threads to remove them. If they just didn't thread the barrel end you're golden! Measure the barrel end or see if your new muzzle break won't slip on. If it doesn't then it was never threaded. Beg,borrow,buy or rent a die and guide. Lil' cutting oil and a few twists of the die (counter clockwise) and you're done. Look at ak building web sites and you'll find the die, or ask to borrow one.