Aha! does not allow importing of custom fields at product level. This will be useful if that can be supported.
Thank you for your idea. Aha! provides the ability to import specific records via CSV such as Features, Ideas, etc. The focus is on records where you have a very large number that need to be created. Custom fields are created at the account level and then applied to specific products using Layouts. This is intended to streamline the process so that you don't have to create the same custom field multiple times for each product.
Give the current capabilities and other priorities, we are unlikely to implement this idea at this time. We hope you understand.
Agree that bulk updates of fields at the product level would be helpful - if not by import than by a bulk update in app.