Looking at the
source code (with my limited programming knowledge) I think there is a pretty definite list of exiting meta tags that can be imported (with various possible syntaxes). But I also found out there that you can freely import IDs. Something like this works:
<meta name="dc.identifier.url" content="http://www.abc.xy" />
<meta name="dc.identifier.website" content="http://www.gkl.xy" />
<meta name="dc.identifier.slartibartfast" content="http://www.thhgttg.xy" />