-does that mean i can use those textures in blender when i'm making the map?
Unless their seamless, yes...
-if so where can i get these textures?
Check out the .pk3 files in the baseoa/ directory. They're actually just .zip files
-if i included other (gpl) textures in a map that didn't exist in openarena would i be able to bundle them in the map so that it worked in the game?
Sure! If you mean, adding custom textures; this works since Doom 1 (or even Wolf3D?)