Merged Fields

I have to assume by "special codes" you mean specifc addressee/salutation types that are NOT "Primary Addressee/Salutation." If that's the case then in the mail module you're setting up, do the following:

1. Go to Fields to Include tab
2. Go to Addressee/Salutation > Additional Addressee (or salutation, as the case may be)
3. When it asks for Number of Additional Addressee fields to export, you want to go with the default of 1
4. Hit OK
5. In the Customization Options screen, select the option "From individual", and then use the first dropdown menu to find the specific addressee or salutation you intend on using
6. The second field will allow you to default to something else if that specific addressee or salutation type doesn't exist on a given record.
7. Finally, just make sure you're using Additional addressee (or salutation) in your actual merge document.

If that's not what you mean, please give us some more detail about what you're trying ot accomplish.