I'm not deep into PowerDraw yet (only just started playing with it last night), but I would suggest something like this: render the lights as white-to-black gradient spheres on a VScreen, using additive blending; you don't need alpha for this, since you'll blend them additively. What I would do then, is draw that entire VScreen onto the main display (after rendering your background tiles or whatever you have there) with multiply blending, so that areas with no light in them get dimmed out.

The special lights (e.g. for toches and such), I would render afterwards, for example as yellow-to-black gradient spheres with additive blending (again, no alpha necessary) ...

Not sure if it's at all possible to draw a VScreen onto the main display with filtering options though - although I suppose it ought to be? ... anyone?