Since the Overlander is built on an Android tablet, you could easily (relatively...) sideload the OB Android app on it. Now that you mention it, I need to try it... I have the GAIA GPS and BRMB (Back Road MapBook) apps installed on my Overlander and they work pretty well.
The Overlander also has a web browser, so you could access the OB website that way. I did that once at a trailhead when I realized I had not uploaded the correct route into my Overlander. Create a WIFI hotspot with your phone, connect to it with the Overlander, get to OB on the web browser and download what you need.
To create routes, waypoints and such, the Overlander uses Garmin Explore. You can fairly easily create what you need on the web based app and then sync to transfer what you created to the Overlander. Long routes are a pain to create in Explore, so I personally use GAIA and then import into Explore to get it on my Overlander.