Layering Normal Maps for use in Daz Studio

Hi!

I was wondering if there is an easy way to layer two normal maps together. One is from a commercial package, containing "broad" details such as a bit of muscle definition and veins and extra skin details like wrinkles. The other is one that I made myself, containing a lot more details like pores and such. Yet I wanted to layer both together in such a way that the one I made myself, containing all the details "follows" the one with the "broader" details: when a vein runs trhough that map (the "broader" one), I want that elevation and depression to show up on my own normal map too, without losing the pores I painted in that particular area. Can this be done? If so, How? I have access to both Gimp and PhotoShop to pull this off.

Thanks a lot,

Kind regards,

Me