The migration pathways can be edited if there is a good reason to, and having a lot of overlap with another site and having a lot of questions that should be migrated over are usually a good opportunity to open a dedicated migration pathway -- we have one with Math.SE, for example.
The guidelines for migrating are generally:
- Don't migrate crap
- Be certain the question is on topic on the target site
- Don't migrate questions that are on-topic here
These are related, of course. If one is not intimately familiar with the target site's policies then it is easy to end up migrating something that is not good quality for the target site.
There is a corollary to the guidelines:
- Avoid migrating to beta sites
The idea here is that larger sites shouldn't be sending large numbers of questions to a site that is not yet on its feet. It is up to the beta community to find its niche, its voice. So flooding it with migrations from outside is frowned upon.
For the case of HSM.SE, they are still in a beta stage. Additionally, we don't have a large number of questions that would be good candidates to migrate. So I think the way to suggest a formal migration at this point is to raise a custom flag and suggest migrating it -- we'll take a look at it.
In fact, it appears formal migration paths aren't even allowed for beta sites and it will always require moderator approval.
However, the caveat to that is I don't know if any of the moderators are intimately familiar with the scope and culture over at HSM. I personally would decline flags to migrate unless it seemed super obvious to me, in which case I would try to go talk to the mods over on HSM to see if it would fit in well there.
The best option though would be to leave a comment and suggest they post their question there instead, while also voting to close it here (because good migration candidates are questions that are not on-topic here). OP can either edit the question to make it more on topic here so it can stay open (and become a not-good migration candidate), or they can go ask their question on the other site.
The comment will hopefully introduce them to a new community that they didn't know already existed!