Change schematic to a hierarchical sub-sheet

I have a nice schematic sheet and want to use it as a sub-sheet in a root sheet.
There is a “Create hierarchical sheet” icon in Eeschema, but it’s only for new sheets.
The “Place > Hierarchical sheet” similarly only works for new sheets.
How to add an already existing sheet, as a sub-sheet, to a root sheet?

Thank you.

Thanks Andy_P. That worked!