Can I put a grille insert on my F-150 to protect the inter cooler for the turbo without airflow problems or would it void the warranty.

 

Just about any time you alter anything from the original design without manuf approval
you leave the door open for conflict with warranties. Even external dirt can cause problems with intercoolers so It might be wise to consult with a dealer beforehand before placing any type obstruction there. I'm sure the size of the airflow through the insert is a consideration too.