Unless those questions are deemed off topic on those other sites, we have no cause to remove content from another site and the reputation/ownership that goes with it.
Besides, the wholesale importing of content from elsewhere is not really a great way to build a new site. Questions imported from elsewhere would likely be orphaned if the author does not have ...
No, that's not a good idea.
There are some issues with that suggestion:
You can't "pull them in", they have to be pushed over by the other site.
Once they have been on the site for a while they get "too old to migrate".
It is deprecated to ask identical questions on several sites.
Many sites don't consider beta sites as valid targets for migration.