Setting Up Sourced From Codes and Lead Times for MRP Planning

We are a multi-national corporation with three main distribution centers for export. Currently, we have set up three sourced from codes in the Branch/Plant Relationships for all the importing branches. This has a standard lead time based on average transit time. Exceptions have worked because we've managed outside the system and/or carried extra safety stock. We are trying to find ways to use the system to become more JIT.

There are a few issues we are encountering that I'm not sure how to address:

1 - We have product categories that require testing on inbound. Increasing the standard lead times means carrying more inventory for all our products. But, I don't know of a way to add additional lead time to an item with a Sourced From code.