Adding it is really not the problem the bottle hack has been around in the TDI forums for a long time. They fill the "adblue" which is the same at DEF complementary each 10,000 miles for 36k miles. Depending on how you drive that could equal anywhere from 2.5 gallons to 4 gallons.
VW charges about $15-$18 for a half gallon. Let's say for an average currently every 10K miles I'm using 3 gallons. That's $90- $108 every 10k miles. Clearly after the complimentary stage you switch over to the cheaper DEF, which you can get 2.5 gallons for$12- $35 depending on your local availability , so you looking at around $18- $50 every 10k.
Now they are saying that a fix will require at "least" double the amount of DEF and VW is talking about adding a larger tank. So if you have VW fill your tank you're looking at $180 to $216 every 10k at least.
If you do it yourself you're still looking at $36- $100 every 10K at least.
If you're really brave you could pull up to a Loves truck stop with your car and fill up a gas can and have the truckers yell at you while you're filling up.
So you have the added cost of DEF and the added cost of lower fuel mileage.
WHO PAYS? Now you know why lawyers around the country are circling, massive criminal fraud on VW's part.