Each Stack Exchange site has a limited number of "default" migration paths (those sites that show up when you vote/flag to close as Off Topic > Belongs on another site). These paths should be reserved for those sites for which there is a proven need for fairly frequent migration. I'm not sure if too many "Stack Overflow questions" get asked here, or how frequently such migrations are successful, though others could look into the statistics.

For information about when migrations paths should be set up, see the following on Meta Stack Exchange

* [When should we consider adding a default migration path?](http://meta.stackexchange.com/q/261196)

My feeling is that there such questions are rather infrequently asked here, and it is not worth it to add Stack Overflow as another default path.

If you feel that an SE site not among the default is the appropriate site for a question, you can use a moderator-only "in need of moderator intervention" flag and describe your request. Site moderators can migrate questions to any other SE site.

Such questions can also be closed simply as off-topic ("This question does not appear to be about research level mathematics within the scope defined in the help center") possibly with an accompanying comment directing the asker to a site which might be better suited. (Unless you are very familiar with the target site, I highly advise against using definitive phrasings such as "This question belongs on....", and would suggest instead something like "This question might be better suited on....".)