Automatic & Selective Enabling of User Accts

Hi I need to be able to either

Enable a user for archiving with a powershell Script (we have archiving tasks / mailboxes over two mail servers)

Use a AD attribute (preferable a cumstom one) that the auto enroll function can detect then enable the account for archiving.

Basically my problem is that we have numorous service / special accts with mailboxes we do not want to push to EV, but currently have no way to distinguish them which is where the custom attribute would help. in my ideal situation i would like to sweep AD user objects once a week and enable any objects with a certain attribute.

i could even script this if i could get a powershell command to enable the MB for archiving, we are using EV10 with exchange 2010 at the moment

Thanks