There are detachable daypack backpacks, like
these, but I can't see anything at the sizes you're after. To be honest, if I had 30-35L to carry, I'd just take the 80L bag half-empty. I don't see how you get a 35L pack both compressible enough to pack, and with the support to be comfortable for a day on the hills.
Looks like 'stuff pack' might be the term to look for - ie,
here. But from the look of it, you might as well rope a Tesco's bag for life to your back.