The current version of the graphics editor for SGDK2 isn't sophisticated enough to be able to handle something like that. I imagine it wouldn't be too hard to detect something like that, but there are programs made specifically for drawing with pen tablets which can easily export to an image type that SGDK2 can import, like Gimp, Photoshop, and Painter 2.0.
However, since the development "team" is only one person really, and he doesn't own a pen tablet, testing would be difficult, and since most people who own pen tablets have other graphics programs which are much more advanced, I don't think it's going to be a high priority.