Duergar has been introduced as a PC race in Monster's Manual 3 if I'm correct. I have found no sight of additional Duergar rules if any other book for D&D 4th edition. I was wondering if Duergar has been designed so that it can be treated as a Dwarf when it comes to acquiring racial feats, racial paragon, or epic paths.
No. As with other monster races that did not receive a Dragon Magazine article, Duergar do not have additional support. They also never received a flexible stat (most races were updated to get +2 to one statistic and +2 to one of two others, but Duergar are stuck with Con and Wis only). And because they aren't explicitly able to access Dwarf feats, they can't do so.