This is going to be achieved with talking with the engineering types, and getting them into the habit of giving Purchasing some of the work that engineers normally do. This would be things like calling suppliers for prototype piece-parts, gathering specifications of materials, etc.
For example, an engineer might want to spec 4130 steel for a particular part, a material that is not normally used at the company. Engineering should get with Purchasing and let the purchasers work through the efforts of contacting a vendor/source and haggleing prices.
The only draw backs are that while Purchasing is following one white rabbit down one hole, Engineering might decide to chase a different rabbit down another hole. Usually when this happens, Engineering never tells Purchasing that the requirement has changed, and this leads to wasted time of both parties. "The attempt and not the deed confounds us."