Items file reference PaperUI created items?

So I’m a fairly versed OH user (1.x not as much yet on 2.x). I’m looking to do some advanced things with tags/groups to enable support of groups of switches defined inside of OH that can be controlled from Siri (homekit binding) and Alexa (hue emulation binding). It’s a pain configuring and setting up the settings individually on these systems and then when refreshing things having to do it all again. I’d like OH to be the one source of config.

Now my issue is, I’ve gone this go round with configuring all my items/things through PaperUI and HABmin. I’m trying to stay out of text files unless I really have to. Unfortunately, I think I now have to. My problem is, I don’t want to break any of my items already created, but I’d like to do some group creation/specification in an .items file.

Can I functionally specify/reference a Item that is already created and configured in an .items files?
Is it possible at all to specify tags on a Group only item?

Does this seem like it will work appropriately to do something like:

Group     All
Group     DownstairsLights     "Downstairs Lights"     (All)     ["lighting"]

Switch     HallLight     "Hallway Light"     (DownstairsLights)     ["lighting"]
Switch     KitchenLight     "Kitchen Light"     (DownstairsLights)     ["lighting"]

And what will happen if Switch HallLight and Switch KitchenLight already exist as items? Will it break the currently configured items, not even recognize them, or just merge the group/tag specifications?