OK, so I did some digging and found the answer myself. In order to add a blank member in a dimension override, simply put ,"" in your formula. For instance, here is a member list formula that includes a blank member:
D9&","&D10&","&D11&","&""&","&""&","&""&","&""&","&""&","&""&","&""&","&""&","&""
Where D9, D10 and D11 are id's for members in a time dimension