As far as I got it, this is not a fixed/defined process (as this would be likely exploitable) - it is possible by watching network traffic to learn about current and new validators though, so you can already have some pubkeys to start a search.
A suggestion was that some kind of "hubs", "aggregators" or "collector websites" should be built where people can post their public validation key(s), some additional info (e.g. who they are, if they have special policies regarding what they will vote on or whom they include in their UNL...) and then sign this with their private validation key. These signed statements then could be used to (semi)automatically build your private UNL.
but what if the majority of existing UNL dont wish to include you can not just opt in like BTC/POW or Peercoin/POS as I understand.
This currently is the case, as RippleLabs atm. does not add third party validators as far as I'm aware.