I've read this article which tells me how to restore a frozen bucket.
When I restore a frozen bucket, can I select which index it will be restored to? How do I do that?
That would be determined by what path you put it in when you thawed it.
/opt/splunk/var/lib/splunk/defaultdb/thaweddb/ would be main
/opt/splunk/var/lib/splunk/cisco/thaweddb/ would be cisco
/opt/splunk/var/lib/splunk/whatever/thaweddb/ would be whatever
At least if I remember right.