I know there is an Import Record function built in to Sage 50, but I understand that it will only add customers, and not update existing customer information.

Is this correct?

I need to update the contact name (and addresses) for 1500 existing customer accounts and maybe add a few new customer accounts as well twice a year.  Can this be done within Sage 50, or do I need to use a third party utility.

If Sage 50 doesn't allow this, why not, it would seem rather easy to allow changes to accounts, along with adding new accounts.