Okay, let me get your current setup/needs right:

You have multiple rooms with dedicated Raspberry Pis, that each run PiSignage to display automatically forwarding (google) slideshows.

You now want to minimally change this setup to allow people to manually forward slides.

This begs some questions:

1. How are the slides selected/uploaded?
2. How are the slides forwarded?
3. How does the setup know if it’s an automated slide or not?

In my head the new setup would look something like this:

The Pis stay, as does PiSignage.
A device is added to forward slides (most likely a Bluetooth remote)

Here is where it gets tricky.

On remote press, a menu could be opened, to select uploaded slides and display them via other means than PiSignage. Closing this slide opens PiSignage again.

But having this easily maintainable is tricky and it will get hacky and people will forget closing their slide and so on.

Alternatively USB-sticks could be used.
Inserting one opens the folder, a slide can be selected with the remote and removing the stick opens PiSignage.

Both methods are hacky and not easily maintainable. But I can not think of other means.

Also I think that you should first think about some means of uploading and selecting slides as well as whether you even want to keep using PiSignage.