Caster is not usually adjusted, I don't think. I'm pretty sure that if caster was out, you'd have bent parts. Replacing them will get you back to stock.
Moving the upper strut bearing, forward or back, will adjust the caster, but from what I understand there isn't enough room there to make any significant change. It's not like camber, where 1/2Âº is a noticable change. I am not an expert on alignment, so caster could have a bigger effect than I give it credit.
Who is checking your alignment for you? They are able to tell you that caster is out, but are not telling you how to fix it? What did they measure your caster at? Any idea what type of equipment they are using? Are you sure that you can trust their equipment?
I would also guess that any damage that knocked your caster out, would have also screwed up the toe, but I guess it's possible that it didn't (just very unlikely).