Is there any way to pay transaction fee in XEM from selected account as mosaic owner, for all transaction fee made inside certain mosaic so users would not need 2 currency or mosaic: your (if you have levy) and XEM (and to recover those cost in XEM by levy if you want it)?
It should be possible using next version of blockchain called Catapult.
This feature is called Aggregated Transaction (https://nemtech.github.io/)