Say I'm using the Grassland tileset and want to use a tree from the Woods tileset, is it possible to import that tile, or is each map limited to the given tilesets?
You have to export both tilesets so you can get the .png files, then you have to edit the grassland tileset, make it big enough to accommodate the tree, and copy and paste the tree from the Woods tileset. When you've done that, import your new tileset (you can leave it as the original name), then set the passabilities on the tree, and you're right to go.
Just be careful with the copy/paste - the tiles must be in multiples of 32.