If you already have a Windows 11 VM in another format (VDI, VMDK, RAW, VHDX), use qemu-img .
Microsoft does directly provide Windows 11 as a ready-to-run QCOW2 file. They provide ISO files and VHDX images for Hyper-V. To get a QCOW2, Alex had two official routes: windows 11 qcow2 download
Which are you using (Proxmox, pure QEMU, Libvirt)? If you already have a Windows 11 VM
If you absolutely need a ready-to-run image for testing, consider (90 days) from Microsoft, then convert it yourself. consider (90 days) from Microsoft