Insert Records that exist in one table that are not in another

I have 2 tables. One is a master table and the other is a temporary table that is used for importing data. There will be times when the import file will contain a part # that we do not have in the master table, so it will need to be added. Below is an example of how I would need it to be setup.

PartNumber	ImportTable
A1		A1
A2		A2
A3		A7
A4
A5

With the above, I would need A7 to be added to the main table. Any assistance would be appreciated.