This is not possible right now. Digital copyright protection will inhibit this. Broadcasters will not allow users to download content to their PCs via USB or firewire, and companies such as Motorola and Microsoft are not enabling those ports due to pressure from operators and broadcasters.
It would not make business sense for them to do so.
Until they have perfected (or nearly perfected) digital rights management for digital content across all industry players, then don't anticipate they'll allow customers to simply plug in their USB drives to their DVR units.
If you can find a good hack, thats a different issue.