Yes it's possible, this is how I do (usually)
I take from the History tab, if it's not open go to Windows ➤ Dockable Dialogs ➤ Document History (if the document is not listed just open it, then close it, it will be listed)
I click (and stay clicked!) on the .xcf I want to insert in the opened image and drag it where I want in the Layers stack
In the screenshot below, you can see that thin black horizontal line ➤ is where it will be dropped when you release your mouse click
I also do the same way (drag 'n drop DIRECTLY in the middle of the layer stack) with any images on the desktop or directories/folders, it works the same way
I take from the History tab, if it's not open go to Windows ➤ Dockable Dialogs ➤ Document History (if the document is not listed just open it, then close it, it will be listed)
I click (and stay clicked!) on the .xcf I want to insert in the opened image and drag it where I want in the Layers stack
In the screenshot below, you can see that thin black horizontal line ➤ is where it will be dropped when you release your mouse click
I also do the same way (drag 'n drop DIRECTLY in the middle of the layer stack) with any images on the desktop or directories/folders, it works the same way