Following this discussion about auto-adding a new tag bibtex-entry after importing bibtex in the RefNotes plugins.
actions in the widget $dropzone  is triggered after converting bibtex into tiddlers, and before display the import button on the import tiddler.
In this step, the json tiddlers are generated int he import tiddler, for example
{
    "tiddlers": {
        "paper_1": {
            "title": "paper_1"
        },
        "paper_2": {
            "title": "paper_2"
        }
    }
}
Not I am not sure how to add a new field tags into each tiddler.
$action-setfield with $index can only add field in the root of json.
<$action-setfield $tiddler="ImportBibtex" $index="tags" $value="bibtex-entry"/>
So my question is how I should add a new field in each tiddler? That is
{
    "tiddlers": {
        "paper_1": {
            "title": "paper_1",
            "tags": "bibtex-entry"
        },
        "paper_2": {
            "title": "paper_2",
            "tags": "bibtex-entry"
        }
    }
}