Adding additional block devices to XEN guests is accomplished using xm block-attach command. In this example I am exporting an external USB hard drive that was recognized as /dev/sda1 by the hosts kernel. Important thing to keep in mind is that you must use xvdX as the guest block device name.
xm block-attach 6 phy:/dev/sda1 /dev/xvda1 w
This command will export /dev/sda1 on host machine to guest domain 6 as /dev/xvda1 with write permissions.
You can also add/remove NICs on the Xen Virtual machine . Refer to http://unixfoo.blogspot.com/2009/04/xen-adding-and-removing-nics-on-virtual.html