Yes, you can. 
Add the images (and if you have, pdf files) >>> into the album folder. That’s btw something I recommend to do with the main cover, too – that way the work you put into cover art is easier to back up and also usable outside of Roon.
Something like:
front.jpg (or .tif(f) / .png) … for the front cover (this would also be picked up by Roon as the cover image)
back.jpg … for the back cover
…
sleeve.jpg … for the record sleeve
…
booklet.pdf … for booklets
inside the album folder will work out fine. Roon will list additional pictures and pdf files on the album page.