By "standard d20" do you mean "standard DnD" ?
Setting aside for a moment the question of just what the hell "standard DnD" is. I dont' think that would be a good idea because DnD makes a lot of assumptions about how much magical power and how much gear a character has and Conan's assumptions in that area are very different. You can definetly mine conan for some individual sub-systems (class defence bonus/armor as DR/lowered massive damage threshold is a good example) but I wouldn't advise trying to use the Conan rules in their entierity to run a game of DnD.
If on the other hand by "standard d20" you mean a swords and sorcery game set in some other setting besides Hyboria then the answer is absouetly yes! Conan is probably one of yoiur best options in that regard. Probably the only work you would have to do is to make up new races for your campagin setting, maybe tinker with the equipment section some in order get the flavor you want. It is very easy to remove the Hyborian flavor from the book and be left with an excellent swords and sorcery, pulp adventure game system.
I seem to remember that Iron Chief was tinkering with porting Conan to a Farfid and Grey Mouser game sometime back. Maybe I can find that thread later.
Hope that helps.
