Question: add-face-text-property for 'font-lock-face?

[Qiantan Hong]

Users of crdt.el report that crdt-visualize-author-mode (which use 
font-lock-face to color code author of texts)
conflict with faces from other modes. Apparently there are more than 1 mode 
which sets this text property.

Is there some way to add-face-text-property to 'font-lock-face property, 
i.e. adding new face attributes instead of overwriting any existing face?
add-face-text-property doesn’t seem to work because 'font-lock-face will just 
override ‘face.

Best,
Qiantan