OK, looks like there's already a good way to do this -- but not on a beta site!
Flag the question
Select "It should Be closed for another Reason"
Select "Off Topic"
Select "It belongs on another Stack Exchange Site"
Then it gives you a choice of a few alternatives which seems to be automated based on keywords in the question or something.
I think Ethereum would be a good candidate for a migration.
There is no ripple SE site, perhaps you meant Monero? Note that Monero is still in Beta and thus can't be added as a migration unfortunately.
Another good migration would be to crypto.SE in my opinion