It is not possible to define custom metadata in Fluid Topics with Confluence labels.
Fluid Topics supports custom metadata with Confluence content through a control file.
When uploading the FluidTopicsControlFile.xml file after having uploaded the Confluence content, use the document's originId to target it.
For example:
<?xml version="1.0" encoding="UTF-8"?>
<controlFile>
<resources>
<resource lang="en-US">
<originId>TEAM</originId>
<metas>
<meta key="author">
<value>John Doe</value>
</meta>
<meta key="editor">
<value>Jane Doe</value>
</meta>
</metas>
</resource>
</resources>
</controlFile>