Moses, you should be able to use a standalone TiVo with your DirecTV unit, and the standalone TiVo would have wireless capability. However, I think there's other considerations, e.g. whether you'd get the TiVo guide or the DirecTV guide, how many tuners you'd have, how upgradeable/hackable the DirecTV combo unit is, etc.
I don't have DirecTV so I don't really know any more about it, but you can ask over at
http://www.tivocommunity.com
Edit: another factor is how the standalone TiVo would control your DirecTV box (to change channels) - whether you could use a serial cable connection (if enabled on the DirecTV box) or have to hang IR emitters on the DirecTV box (not so elegant).