we have been asked this several times, so here a public answer:
The XTension interface between QuarkXPress 2015 and 2016 was the same, which means – if the developer of the XTension didn't force a version check – you can use a QX2015 XTension inside QX2016 and it will work.
QuarkXPress 2017 will have a slightly different XT interface (programming interface). Meaning you cannot simply use a QX2015/2016 XT inside QX2017.
So you need to ask the vendor of the XTension for an updated version please.
Good news are, we kept the changes minimal, so development effort for porting should be minimal, testing effort of course could be larger for our partners, so best to check with them, when and how they can provide an update.
We already know of many partners who will be ready and offer their updated XTensions when QuarkXPress 2017 ships.