Willie --
Not really; the Kindle "Metro" app on the RT will not allow it, but, since you are running a Pro, you can install the Kindle for PC app, which does allow sideloading. That is what I am using, and am happy now because I can continue to work my way through my library of .mobi books.
Baen uses PRC or MOBI (essentially the same), plus some others; the Kindle for PC app will read it. I just downloaded a free book from Baen, and it came in .mobi format. Loaded it into my Kindle app and opened it. Again, I am using the "Kindle for PC" app on my Pro, not the app that was written for RT.
Take care,
Russ