Joined: Aug 2010
Posts: 12,760
Likes: 2,117
From: Madison, WI
Bikes: 1961 Ideor, 1966 Perfekt 3 Speed AB Hub, 1994 Bridgestone MB-6, 2006 Airnimal Joey, 2009 Thorn Sherpa, 2013 Thorn Nomad MkII, 2015 VO Pass Hunter, 2017 Lynskey Backroad, 2017 Raleigh Gran Prix, 1980s Bianchi Mixte on a trainer. Others are now gone.
If you have two panniers on the back of your bike, I see no reason why you can't just strap a 30 liter backpack on top of the two panniers. Some of my bike tours I strapped the Ortleib 31 liter duffel on top of the rear panniers, I would expect a backpack to strap on top in a very similar fashion.
A couple photos of my 31 liter duffle on top of the rear panniers attached.