yes, but it is not officially supported cause one of the other o/s can modify the qubes boot partition, or has better chance to infect bios or firmwares. It also takes more tweaking to add other os entries to the qubes grub, because I believe it has no os-prober.
I found it easier to add my qubes entry to the trisquel grub instead, in the /etc/grub.d/40_custom file. In there you paste the XEN section copied from the qubes boot partition /boot/grub2/grub.cfg file. then update-grub. Probably should repeat the step when qubes updates its kernel or initramfs.
An easy way to reinstall the trisquel grub after installing qubes is here https://community.linuxmint.com/tutorial/view/245