I hate the idea of putting horrible self-tappers through my mudguard so have always glued extenders on with silicone sealant. I dare say there are plenty of other adhesives that would do the job just as well, just clean the mating area beforehand and roughen with sandpaper to get a decent key.
I didn't bother taking the mudguard off when I did the Fazer, having the wheel in place is handy as you can jam a bit of crumpled newspaper or rag there to maintain pressure while the silicone cures. Gives a nice smooth result and has stayed put for 14 years of all-weather use so far.