Convert tag to custom field

I have a large body of posts that have been imported. They are tagged with the writer of the posts in a format of ‘firstnamelastname’ with no underlines or dashes. I’d like to automatically convert those tags into filled into a custom field with the name ‘Writer’. Is there any way to end up with each post with a custom field of ‘Writer’ and the name formatted correctly as ‘Firstname Lastname’?

I’ve looked at things like Taxonomy Converter, Types, Category to Tag converters but none of them seem to do what I want. Any suggestions?