Authentication either username or mac address

I have searched about this, but couldn't find proper answer. If it has been already answered kindly share the link. Would like to authenticate users either with username or specified mac address. Generally we can authenticate users either when using only username or only mac address (using mac authentication). But let's say we want to give users the option of either using username or mac address to login, but count data and session time on username basis, how to achieve this in freeradius? - Mohammad