Sorry for the delay getting back to you.
It’s not currently possible to bulk assign customers to a site or bulk update the “Use only linked sites for deliveries” field on a customer, but these are both great suggestions which I’ll add to our feature requests list.
Can I ask - is your reason for using this feature because you’re needing to keep packaged beers separate for “quarantine”, conditioning or something similar (as per this)? If so, we’re going to be building a specific feature for this soon which might be a better option here (if this is your reason for doing this at least).