How to add existing flow to a device?

Well, actually, you can!

Use the App Device Capabilitities.
In the App settings, you can export (advanced) flows.
Then press create TEF.
Then click on import and Read.

Now you will get a list of devices, select the new device from the dropdown/list shown for the old device.
Then press Update script and run that script in the dev (follow instructions).

Now you will have a duplicate from the flow in your root folder, with all old devices (and tags) replaced with the new devices.

I use this to copy flows for all rooms that have the same setup (kids rooms).

Works great!

See here, i have documented everything well enough at all these links:
General link:
The Flow Exchange(r) - Exchange Your Flows with Others! - Flows - Homey Community Forum

More specific import link, and specificly what you need:

And as states:
This can also be used to copy/paste flows on your own Homey, and change devices, users, etc., in very fast way!


The Flow Exchange(r) already works great for exporting en import between users, but using it only local, in my experience, works perfectly!