No such luck...
The only adapters that I have seen are for converting a regular thru-axle 20mm hub into a QR.20 (Marzocchi) style hub.
You can't easily convert a regular hub to the QR.20 since the spacing on a QR.20 hub is wider than a regular front hub.
Your only really choice is to build a new wheel using a QR.20 hub (or a regular 20mm thru axle hub with the Marzocchi adapter). And as you said, if you were to adapt a regular hub to fit your fork, you'd lose the increased stiffness that a 20mm axle hub/fork combo gives you.