virtual MAC addresses?

If I were you I would just disable the MAC filtering on the router - it doesn't significantly enhance your security, and in this case, it's just creating a headache. The reason I say it doesn't enhance security is that it is very easy to bypass. Let's look though at this "virtual MAC address" concept - which quite frankly I would see to believe. IF your extender is functioning as a repeater (the extender itself connects to the router via wireless), it will/should substitute it's MAC address for the client's MAC address as/where necessary - as a repeater, the client has to send data to the extender, and the extender then repeats or retransmits that to the router, and the router must then reply to the extender which must then retransmit to the client - the data get's to it's destination based on the MAC address, so, since the client never connects directly to the router, what the router will/should see for all the repeater clients is the extender's MAC address - so adding that one MAC address should suffice.