Can we add more KPI item(s) - related to safety matters - to contractors after the contract is awarded and signed ?

Yes, contract is always good thing to add documentary proof

KPI -  key Performance Indicators can be added as per company laws and mutual agreement. This is going to benefit to the company but for the contractor if this is going add some cost then he may hesitate to add. But  you can always talk to your contractor and come for a special arrangement for changing the KPI.