It isn't impossible however, it also is quite difficult from what I've heard for people to trade into
DVC who do not own DVC. That said, it may depend on which DVC resort they're trying to get into, size villa they're trying to get and time of year they're trying to go.
Busy times for DVC tend to be quite different from the non-DVC WDW resorts and some of the DVC resorts are smaller than others. BCV's and BWV's tend to be very difficult to get during the food-n-wine festival at Epcot for instance and Oct.- early Dec. in general can be tricky at lots of the DVC's because it's lower point season and the holiday festivities are in full swing. One more thing to keep in mind is that DVC utilizes a point system where we can go to whichever resort we choose at ANY time of year in any size villa all subject to availiblity and how many points we have to spend unlike most time shares that have a set week. I don't know how that does or doesn't affect things as far as how flexible it would be for your inlaws to book

So as you can imagine, if it's difficult for the members who already own at certain times, I'd think it would only be more so for non-members.
I'm mearly passing on info. that I've read of other's who've either asked this question before or commented on the subject in the past. If they can be flexible about which resort & dates and even the villa size it may make all the difference but since I actually have no experience I really couldn't say

It must not be totally impossible to be done though or it wouldn't be an option. I'd have your in-laws start inquiring with their timeshare ASAP though and go from there...you'll never know until you try.
Best of luck and let us know how you make out
