You can use a tool that converts a physical machine into some well known
VMM format, such as to VMWare Workstation VM format (VMWare Converter is
the name of such tool, IIRC), and then you can use qemu-img to convert
the resulting VM disk image file (e.g. *.vmdk) into the "raw" format,
which is the one used by Qubes for its VMs.
Once you have the final image (call it root.img), create a new
standalone VM (qvm-create --standalone) and then replace the root.img of
the newly create VM (/var/lib/qubes/appvms/<your-appvm-name>/root.img)
with the root.img you created in the steps above. Make sure the VM is
not running when you do that.
I tested this for VMWare VMs and it worked fine. Note: the VMs you
convert to Qubes should not have any custom VMM extensions installed,
such as VMWare Tools, or they might crash during boot.
joanna.