If you're just starting out archiving, and you want to add certain priority users to the system first, then you can do a staggered rollout using Provisioning Groups.
But if you're instead looking (as I suspect) for a general, ongoing prioritization, like
"Run every night for these 1,000 users, but always make sure to do these 50 users first and, if necessary, at the expense of the others"
then no, I don't know of any way to do this.
--Chris
