Since you mention a Z-partner function, but not customer hierarchy, I assume that this is a partner manually added to the customer master of the sold-to party, which you populate in the sales document (or you derive it from some other source and add it to the sales order partners).
In this case the procedure is very straightforward - have you tried to search in SCN?
When you enter 'SD add field pricing', it produces a lot of results that you can filter by asset type.
The first result from the wiki page subset includes step-by-step instructions with an example on how to add a new field to the price catalog and populate it.
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