As it now calculates which effects stack rather than using a preset option, you'd need to make some alterations to power effects, specifically:
Make sure they're set to be stackable
Make sure they're set to affect 'self', not 'target' or 'target (also affects self)'
Enable scaling for the power, and that should do it.
I may be tweaking the system to make it overridable, but do bear in mind that everytime the database is updated, it'll replace your modifications. I'll see if I can sort out some form of override to make it a bit easier to do.