The VCard method but you can take a few liberties with the file structure. Reading the VW and Skoda threads and Seat it's a bit touchy about where you put them on the SD card and the format of the card Fat32 would be fine, think people might have Apple issues. There are German and Skoda support sites, the Skoda one is in English which may be useful for generating Vcards on an address by address basis if you aren't bulk importing them.
I've got a file of 100 or so destinations ready to import. Now I call these destinations some people call them POIs. If destinations it's the VCard method.
Basically the standard Ateca Navigation unit is the VW Discovery Navigation whilst the Ateca Navigation Plus is the VW Discovery Pro, so any thing you see working for these units will work for what's in the Ateca.
I won't get my hands on my car for another five weeks or so, the first thing I'll be doing is reading in my destinations and the second thing is trying to upload speed cameras. Anything you read from Leon owners is applicable bearing in mind the unit differences. Basically the speed cameras will import into a Plus unit, not so sure about the non plus. For the latter they either go onto a piece of internal memory or on the card. German Ateca board seemed to be saying they were not getting a "ding" but they might not have been doing it correct. It works on the VW Discovery Navigation Pro as per the Ozzie thread I linked too.
If experimenting I'd start off small with a few lines to test out... that is what I was going to do and do one of those simulated navigation drives. Will wait to hear how it goes . I do Europe trips so a connected approach via Google isn't something I'd do for data rates and not spots. I check the travel time to the destination via Google maps and delays then jump in the car. Might check again if there is a delay.