I forgot to add some things on import. How can I mass update users?

For example:

I imported all of my members and forgot to add their telephone numbers on import. How can I go back in and mass add telephone numbers? I don't see an option on import to do it again or to update on import. Will I have to do this through the mysql database? If so, what do I need to do in there?

You need someone to write a script which goes through the entire data set and compares the entries and updates the missing values where the comparison matches. 

2 minutes ago, opentype said:

You need someone to write a script which goes through the entire data set and compares the entries and updates the missing values where the comparison matches. 

I can't be the first person who has needed to either overwrite or add missing information from an import... I imagine people create custom fields on profiles and need to mass add data to those fields for all their members. Has someone already written a script for this or is there an app for this by chance?