Currently we have no way of migrating Excel questions anywhere. They would be off-topic in most of the cases for http://stackoverflow.com, so we shouldn't migrate them there. However, it seems that https://superuser.com/ is a place where lots of Excel questions get answered -- maybe we should be able to migrate such questions there?
Only moderators have the ability to migrate questions to any site. SE also imposes a limitation on the number of default sites made available in the migration dialog.
The solution is to flag the thread and recommend a migration target.
(In controversial cases, mods can contact mods at the intended destination site to ask them in advance whether they would accept the migration.)
We (at least, we as a site) can migrate questions to SU, (and have, quite recently).
I see this when I choose close/off-topic/"This question belongs on another site in the Stack Exchange network":
Since you don't see it, I am not sure what privilege level this comes at though (20K? diamond mod?) .... [Scortchi says that it's diamond-mod. I believe this recollection is correct.]
You can always close as off-topic and flag. Several high rep users frequently flag to suggest migration* (not all such requests turn into migrations, though - sometimes there are reasons not to -- e.g. it's off topic at the suggested destination; or it's actually on-topic here).
[One such user flagged this post not very long after you commented there.]