After reading this question I was wondering if it isn't a better idea to make a suggestion to the poster to post the question on a different Stack Exchange site instead of vote for migration. Migrating seems to be a difficult task (source) and would mean that the question already meets the rules of the other site, but this doesn't have to be the case.
As a reason for the close vote, allow to make a suggestion to the poster to post it on a different SE site. It allows the poster to decide if the question should be edited to keep it on UX Exchange, or if it should be posted on the suggested site, maybe in a different format to meet the rules of that site. So it still is a close vote but with a friendly message to the poster to think about asking it elsewhere. Often I choose
Off-topic > other just to make such suggestion.
It's a friendly and helpful message
The suggestion is there to help someone to get an answer to their question. It promotes choosing a more friendly and helpful tone instead of just voting to close it as off-topic with a reason but without giving any further direction.
I often see new users post a question that is obviously better suited on a different site. This could make those users feel more welcome.
Provide options to suggest SE sites
Keep it simple and provide an auto-suggest option that search SE sites while you type. We still have the
Off-topic > other option to direct to a different site. Also it should be in Stack Exchange Network's interest to keep people on their sites, right?
So instead of this:
I'd propose something like this:
As seen on in the links above, but it could use a friendlier tone and explain that it is a suggestion.