Can I add a face mask to my helmet? What about a caged face mask/shield?

Yes, If the face mask is manufactured for the brand of helmet that you have. Otherwise, you cannot modify equipment or it may alter the integrity of your safety gear.

Any style of caged face mask/shield (including plastic) is not permitted under section 4.2. Optional Gear of the Risk Management Guidelines.

See the WFTDA Guide for Equipment Check for examples of allowed visors and face masks.