For a GitHub pull request, you specify a base repo (where you want the changes to go) and a head repo (where they're coming from).
However, I would like to use a base repo that is not on the dropdown list. Is it possible to add a base repo to the list? There is a relationship between the two (they share the initial commits)
In the actual repo I want this for, the fork does not have the standard "forked from" that GitHub forks normally have below the name in the upper left. However, it is a fork; it just wasn't forked through the GitHub system. If you know how to add this metadata, I suspect that will also allow pull requests. (This question asks how to change the "forked from". but that part is not answered).