It isn't the textures that need converting but the materials - though granted, if the current materials are very "old school" you may have trouble getting control of things like SSS/scattering. Anagenesis 2 at Daz works with a limited number of maps, so may be worth investigating, or you can just apply the Uber Iray Base shader which will leave all maps (using standard property names) in place while applying the Iray shader for further adjustment; in some cases it may also work to apply a regular Iray skin materials preset but hold down the ctrl key (cmd on a Mac) while doing so and in the dialogue that opens choose the Ignore option for Maps (this will skip texture settings, leaving the existing maps in situ, while applying the numeric values from the preset).