Just for reference, the pre_crosspost_hook error is a bug. You should be able to define an ExternalAccount without having to go into the code and define an External::Site, too. That's what the 'Other Site' option is there for.

Either we need to do a null check there (easy solution) or we need to create a new generic External::Site object for the custom site (a bit more elegant, but would call for more code rewriting).