No, that's actually a valid question--he's just asking it incorrectly.
As the last poster said, Rockbox doesn't have a setting to play with the USB connection types. The specific reason for that though is that the Sansa player doesn't have any connectivity under Rockbox--when you connect the USB cable the Sansa reboots back into the original firmware and connects from THAT.
And in that original firmware the USB connection options are under Settings--USB Mode, like they've always been, where you can choose between MSC and MTP modes.
I'll admit I only know the above to be true for sure with the Sansa 200e series though. I'm betting its the same for the others though.