No, it's not. You can use different mods to make the Ghoul race selectable in character creation and such, but on the original Fallout 3, it's impossible.
One expection would be the Broken Steel DLC (Release date is unknown.), where it continues after the ending, on player's actions. You might be a ghoul if you decided to enter the chamber. But this is completely unsure, so don't keep your hopes up.
If you do the tenpenny quest and help the ghouls instead of persons, a ghoul will give you a ghoul mask...that's not being a ghoul, of course, but is still something if you just like to go around as a ghoul.