You can add postcodes to your LU_POST table via the Post Admin menu item. This menu item will need to be active for the user's Role and enabled within Menu Admin.
Click into Administration.
Click into Post Admin.
Search for the Post Code by typing it into the Post Code box, and click Find.
If the Post Code does not exist, you can add a new one by clicking on New Post at the bottom of this screen.
Fill in the details on this page of your new Post Code such as the Code, address, etc.
Your new postcode will now be within the LU_POST table on your system.