As far as I know, you can connect the Wi-Fi module to the hub through a special connector. Is it planned or possible that someday it will be possible to extend the functionality to the "EnOcean" protocol in the same way using the usb stick?

I don't think that is something they are interested in developing. 1st the cost of redoing a unit, Having external dongles isn't want they want. a Wifi adapter I think is the only exception and I'm sure that wrankles a bit. 2nd is cost of development and potential licensing for something that in our hobby and general IOT world isn't a popular protocol. (Not to say it's not in use in a lot of places, just not on our level). It would make more sense to push development of thread/matter should the need arise. They probably wouldn't object to some who is part of the EnOcean alliance developing something that could do wifi to their protocol and write an open api that hubitat users could take advantage of. Just my 2 cents