On the import's General tab, under What do you want to do?, select Update existing records. In the Options frame, mark the Import records not found as new records checkbox.
Note: When importing new records in version 7, we recommend you also check for duplicates. For more information, review How to check for duplicates when importing new records (BB520394).
The import first tries to match each row in the import file to an existing database record. To find a match, the file must include unique IDs for all updated records. Rows that do not match an existing database record are imported as new records.
For more information, review How to import in The Raiser's Edge 7 — The Basics of Importing (BB58904).