You will need to first get the data into a format of an existing ADU (Account Data Upload). You may be able to use a format you already have or you can consult with support for a recommendation. The DEC format is designed just for this purpose.
Before you run the upload, review your setting for "Status of Accounts with All Inactive Names" (System Preference Administration, Entry Tab, Operation: Accounts. Here you can specify that the accounts should become inactive if all names are deceased or otherwise inactive.