Hello @kll,
Sorry for the late reply, the forum has been quite active these last few days and I’ve been cooking something for you 
I would not recommend the use of addExternalRelation in your case since the data you want to link to is present in your Forest models. The addExternalRelation was designed to enrich data from elsewhere, making the UI treat it as a JSONArray, you would not be able to use any of the Forest Admin UI features (relations, filter, search and so on…)
So we made a new plugin, we hope you’ll like it and find it useful @forestadmin-experimental/plugin-filtered-relationship 
This plugin allows you to create a filtered oneToMany relationship to collection present in your Forest models
Best regards,