Hi Antony,
from what I´ve found out, you can create a "Fill layer" (the little B+ - Icon, afair) and then "fill" this with your existing textures.
Notice, there are some hatched areas, there you can put textures, for example.
So, first import a texture (it will show up in the "self"), then add a "Fill Layer", then assing your imported texture to it.
Hope that helps?!
(Sorry, I got no access to SP at the moment, so the steps come from my mind, tried that on release day of beta)
edit: Oh, and for brush types... it should be possible, to import some "alpha brushes", or am I wrong?
Or do you mean "physical brushes?"
Cheers!