With the eTrex 20, you can just copy any route/track (certainly as long as it's in .gpx format) to the unit when it's connected via mass storage mode. You can copy to the internal storage or SD card this way.
You can also put OSM maps on this way, but it takes a long time (the unit only does some slow ancient USB variant, I believe). I usualy just pop the SD card into a card reader directly attached to my laptop. That's much quicker.
I guess putting the card into a tablet would work similarly.
If, however, you want to connect the eTrex to the tablet via USB, e.g. to download tracks that it has saved into it's internal storage, then you'll need a tablet that can act as a USB host. I don't think that's a particularly common feature (although many tablets will act as a USB device).