This is a tricky one. Yes, you can, but we do not recommend using the Blue Box UV to disinfect facemasks. UVC can only kill microbes on the surface. Phones, glasses, and keys are easily sanitized in the Blue Box UV because they are made up of non-porous surfaces. Face masks are made of textured fabrics that can trap microbes deep within the woven materials, and can potentially hide from UVC rays. UVC rays can only kill what they can reach.
Hospitals have actually started using UVC to disinfect N95 masks and other PPE, but usually in conjunction with other disinfection methods such as Hydrogen Peroxide Vapor (HPV) and Microwave Generated Steam (MGS).