No, there is no way to plugin your own Metadata Processor in Teiid Designer. The only available mechanism for this is in Dynamic VDBs. You can add custom translator to your VDB and try to write your metadata import process.
Or you can build the Teiid Designer and the necessary code.
I already have a dynamic vdb with a custom translator and metadata processor. I was wondering if I could also provide a mechanism to use Teiid Designer against my data source, using the custom metadata processor that I already started.
Now I know there is no shortcut to building Teiid designer with my custom processor.
Loading from [custom] translator metadata in Teiid Designer is something we are planning in 8.x series. So, it is coming, not sure how soon.
If you wish to import your custom metadata into a Relational Model in Designer, we do have a feature that imports XML based on the schema located at: http://download.jboss.org/teiid/designer/7.6/RelationalModel.xsd
See: https://community.jboss.org/wiki/TeiidDesigner76WhatsNew#feature-5 for more details.