Gnashbar has a pannier that converts into a backpack .. pretty cheap ..
so does Ortlieb, + they have a padded back and strap kit
to convert any pannier of theirs that way

I used a Day pack rucksack as the top of the back rack bag , in a tour,

filled it with my stuff and strapped it on , when I needed one,
I took out the stuff bag of stuff, and had the pack..