Add Default Fields To Customer Template in NAV2009

I want to add the default field 31 (Shipping Agent Code) to the customer template (table 5105), but navision does not allow it.

How can I add fields to Customer Template. I can add custom fields with a field number higher then 50000 without a problem.

I found a solution:

Add a field with a custom number into table Customer Template (5105) or better, copy it from the Customer table (18) to keep relations, descriptions and code. Make sure to alter the number to a custom field number

Add this new field to the Customer Template Card Page (5157)

Add code to the CreateCustomer function at the Contact Table (5050) relying to your newly added fields.