This came up on here a little while ago. You might find some more info by doing a search.
The key question here, if such fellowships exist, is whether or not insurance companies recognize you as a dermatologist and reimburse you as such. You perhaps can call yourself a fellowship trained dermatologist, but if major insurance providers don't recognize you as such (and pay you as such) then what's the point? And do you really think other primary care doctors, who also possess a modicum of dermatologic skills, are going to refer their patients to another FP with extra derm training or a board certified dermatologist? Most will probably recommend the classic residency trained dermatologist for their patients with complex dermatologic problems.
Just my $0.02 on the issue: If you want to be a dermatologist, and have the lifestyle perks that come with it, you pretty much have to work your a$$ off to get into the competitive residencies. There are no good shortcuts to this cushy lifestyle through FP.