Yep, this is totally possible. Just choose the linart from editable oekaki you want to use and that clearly states it can be used for adoptables and for pets if you are going to charge for the colorings. Then say "Color me in", next save it and choose "finish later", then go over to the
Adoptable Oekaki board and post it there. Just make sure people know they can color it for free if you plan on charging.
Edit:
I just realized I didn't totally answer. Sorry about that.
You can also find free linarts from other places and use them. Just make sure that you link back to where they give express permission for their use. Also, be positive to read
how to properly link back to the free lines. If you use other linearts and color them on your computer, you would post here:
viewforum.php?f=21