API search for other fields than id_number or email

Hi,

I'm trying to replace an old self-created invoice system with invoiceninja. Historically, the old system is not using integer IDs as customer identification, but alphanumeric ones. I managed to import the customer list, and set the "old" identification as custom_value1. The full text search on /clients is working fine, but I can't use custom_value1 in the API search, which is essential because I need to import invoice items from other databases, where I only have the old customer ID to map the items to the customer.
It would be great if I would be able to do so by
curl -H "X-Ninja-Token: abcde" -X GET "
so I can then create an invoice based on the id_number from the query result.

Is there any way to patch that into the API?