Hi,
You can do this sharing the library with Netbackup, you will only need to create some bar code rules, so you dont use the tapes that BEWS is using, this can be a catastrophe, also ensure you assign some exclusive drives for Netbackup and other for the other system, this way you can control the resources you have and if is posible you can repleace BEWS with a media server and assign the drives to thix box and let the old box be there just has a museum box for restores.
Hope this helps.
regards