Basically:
1) All guests staying in any room/villa on property must be registered. This goes to room occupancy limits and emergency management. Note that there are additional charges for extra adults (real adults, not "Disney" adults) in the room over 2 for non-
DVC stays.
2) Packages (DVC or WDTC) require that everyone on the package be on the same exact package.
3) Dining Plan restrictions require that no "edits" (so, additions, changes, removals) can be done less than 48 (DVC) or 72 (WDTC) hours before check-in.
What CAN work, and what is perfectly legal and acceptable to WDW, WDTC, DVC, and even here on the DISBoards, is adding someone when you check-in. There are a few limitations to this method, namely: unable to use Disney's Magical Express (at least for arrival, they might be able to use it for departure if they're a registered guest by the deadline to sign up for DME, which is like 2 days or so before flight) and occupancy policy (maximum limit and additional charges) must be followed.
I've done this exact method for the past 3 years when Mom comes down for part of my solo trips (not DVC, but same basic thing).
So, overall, it's possible so long as the limitations work for you.