As far as I can tell, this isn't currently possible (in Odoo 16).  I've tried the <group> tag to wrap the new group, and it doesn't do anything; same with using a  to group the inner filters.  I tried using a <div> tag and got an error; same with <menu>, <menuitem>, <Dropdown> and a couple other tags and the way that XML code usually works.

I even dug through the Odoo source code to try to figure out how the date type of filter does this, but no joy.