About 10 years ago I read about someone doing something like this on Linux…
(with Parallels and Boot Camp on a Mac - it was pretty seamless back then)
I followed their guide (i.e. VMware workstation on Linux) and got Linux and VMware workstation to boot a “real” Windows (7) O/S installed on a partition… it wasn’t easy… and in the end didn’t seem worth it… I think I tried the reverse too (boot a Linux partition in a hypervisor running on Windows) but can’t remember how successful…
So - it’s possible… not sure about on VirtualBox though, as I only tried it in VMware workstation…