I'm importing a tab delimited file that has a mixture of new data, that will create a new new record, and old data, thats matched with existing records based on a unique field, that will update the existing records.

But rather then update and more importantly, overwriting the updated data, I want to merge the updated data with the existing data. Is this possible, or am I going to have to create separate records??

Create a separate table that you import into then use relationship between your "data" table and your import table then you can compare your matching records - then a scripted process to import new records from the import table to the data table.