Mass Updating Customers

Is there a way to mass add or update information on customers similar to what we can do in SFDC with Data Loader or the SFDC API? 

We have several customers who essentially are on Comped plans based on several factors in our Databases, but that information wasn't passed into Salesforce. So when the accounts were moved into Gainsight they show as being on regular plans.

I'd like to add tags to each customer that says "comped" or something similar, but I cannot do it one at a time.