When you import, you need to include a column for the existing record IDs in order for it to merge on those record IDs. I typically download the existing records, only include the record ID and any new fields I want to add, and then importing using the Record ID as the merge field. Also, you can use another field to merge on, as long as it is marked in the fields settings as having unique values. Here's a link to Quickbase's help article for importing data as well: Importing Data From Excel Into Existing Apps.
Thanks!
Heather D