Only if you have the Operation: Anchorage DLC. If you finish that early on, you will automatically be allowed to wear Power armor.
You can glitch your way into the Citadel before it's time by climbing a debris pile on the north side and falling through to another part of the map, from which you can run over and talk to Gunny, although it's iffy at best - search youtube, there are videos that show it better than I can explain it. You can also add it through the console. Of course, neither of these is a legitimate way.