New Post: Conditional custom field

Hi,

I would like to add a "upgrade" product to registered customers.

The customers will need to select which of their's product they would like to upgrade (custom list from another DB).

1. I need to add a new custom field (dropdown) which will only show in the "upgrade" product.
2. This dropdown will be populated from an external source.
   

How would you suggest I implement this?

Thanks,
Rony