External tool to migrate from BitBucket Server

Description

I have several BitBucket server (aka Stash) servers running on premise, I have considered switching to GitLab (EE) but need the ability to import all the pull requests/comments etc from the ~20 repositories. This now seems to be supported on the GITLAB side.

But BitBucket server doesn't seem to support OAuth (unlike bitbucket.org)

Proposal

Rather than incorporating this into the service - an external tool that I could run from the command line using manual credentials would be okay and might prove more flexible as this is a one-time administrative operation.