I think you can install Xubuntu with a Live CD/DVD, and choose to place bootloader on your Xubuntu partition and not where your grub is so grub stays as it is. After you finish installing Xubuntu, you can boot into Archlinux and play with update-grub or better use another Live CD or some USB pendrive with Boot Repair on it to fix your dual-boot configuration. Boot Repair can be written on USB with Unetbootin tool. But you need to make sure you can boot into Archlinux after first reboot when finished with installing Xubuntu.

Oh, also make sure you download the right iso image for Boot Repair, they have iso images for both 32bit and 64bit machines.