How do I attribute a subscription to all members without one?

Dear WPMU,

we have the need to bulk import users into our alumni installation, this will mean that when we do there will be some users without a subscription attributed, and by now all we can do is search each one and attribute the subscription plan we want for them individually.

Is there a way to attribute a subscription plan to all the users that have the field empty?

Thank you in advance,
Raul