Alien frontiers: how many fields in a single territory?
Yes, you can have multiple fields in the same territory.

The rules don't explicitly address this (Fields can be found on page 12). However, there is a thread on Board Game Geek where this question is asked, and multiple users answer that it is "perfectly legal," with no dissenting opinions.

Between the lack of explicit ruling in the rulebook, and the validation from other users, it appears that you are fine to place or move fields into the same territory.