Is it possible to load a texture, like a tree bark for example, and use that a Global Texture Mask?

You can paint and fill (with a texture) on the GSM but you can't paste images onto it or merge images down onto it.
You can convert a paint layer to a LSM (Local Selection Mask) and you can have many LSMs which can be separately activated/deactivated.

Depending on what you're trying to do, it may be easier and less confusing to use transparency masks by converting paint layers to tranparency masks.

Thanks for the reply.

Will play around with things and see what I can come up with.