I've investigated this several times and never been able to find a satisfactory solution from commercially-available parts. The main issue that stumped me was obtaining the height required above the barrel to mount a scope - even with high rings.
But I do think it's an obvious way to go with the QB57. I suggested to the factory folk when visiting Shanghai a couple of years ago...
Archer Airguns Inc.
Online stores http://www.archerairguns.com