I am on hold with the MS Store right now. They will stack it (extend for a year) BUT! your subscription will be down graded to the lowest (Personal) level; starting immediately upon the placement of the order. Be careful you don't shoot yourself in the foot if you have a higher subscription level.
The reason I am on the phone with MS Store is to try and get a credit toward the higher level that I have now. Will update.
Update:
It took about an hour, and a half; but was able to stack at a Higher Level (HOME Version = 5 devices vs PERSONAL Version = 1 device) for an extra $10 - but well worth the extra cost for the 4 additional devices.
If I would have blindly Claimed my Offer, I would have converted my current subscription to the Lowest "PERSONAL" Level Subscription - immediately; losing a total of 4 connected devices - 1-PC, 2-notebooks and 1-tablet by doing so.