For example, How I can ask to migrate this post to worldbuilding stack exchange?
You can flag the question for moderator attention (this would apply to non-closed questions as well). See the FAQ about migration; but it also contains a warning:
If the site isn't in the list of available migration targets, or the question got closed without being migrated, first read the below points and consider if migration is/was really the appropriate action (in many cases, it isn't).
You have to consider not only if the question is on-topic there, but also if the answer you got complies with what the target community expects from an answer.
In many cases, it's better just to re-ask the question (and perhaps link to the one on Bitcoin.SE) and adapt it to the guidelines of the other community.