UPDATE:
Based on When should we consider adding a default migration path? and http://meta.stackexchange.com/a/276084/274708https://meta.stackexchange.com/a/276084/274708 it seems like (1) migration paths are not likely to be added while this site is still in beta, and (2) a migration path is not likely to be deemed appropriate until we are sending "tens or hundreds" of migrations to a site within each 90 day period (we are currently well short of that level).
For now, please continue to handle migration requests using the flag feature.
Original post
It seems we can have a maximum of fourwe can have a maximum of four external sites listed, but I couldn't find an option to add them myself. I have asked on Meta.SEhave asked on Meta.SE to find out how to get this done.
For the record, we don't actually migrate that much. Here is a complete list of every question that has ever been migrated from econ.SE.
My understanding is that sites will be added to the migration target list only on the basis of evidence that there are a significant number of migration candidates for that site. Based on the above list, only Money.SE seems to generate a significant number of questions for migration right now so I think it unlikely that sites other than Money.SE will be added to the list.
In the event of a question that belongs on a site other than Money.SE, the relevant response will probably continue to be flagging the question for moderator attention. The mods can migrate to any site and other migration targets can be added to the list if and when we build up a substantial stream of questions that have been migrated to them.