I don't know of a plugin to do this, but there is a trick that might help.
Do your imports, but don't hit the final "import" button. Instead, clone the import dialog, change it to a name like "myimports", then change field "plugin-type" to "plugin" .
Now you can do your import on the original import box.
You can write a quick script to apply your new tag, using a list widget like
<$button>Apply label
<$list filter="[[myimports]plugintiddlers[]]">
<$action-listops $tags="NewTag" />
</$list>
</$button>
I was just thinking of trying Joplin. Why are you leaving it ?
Good luck!