I think that you can either concatenate the special frames to your validation dataloader (way 1, probably better) or loop over the special frames in a post-validation callback step (way 2).
Way 1: Concatenate the special frames to your validation dataloader, literally - add code to the dataloader or (and possibly better from software design perspective) create a dataloader class/specialization which is initialized with the original validation dataloader and first loops over it then over the special frames (or even more, but probably too abstract/unnecesary - create a separate dataloader specialization for the new frames, and a dataloader class which joins any two dataloader classes…)
Way 2: Just subclass pytorch_lightning.Callback, override the post-validation step (on_validation_epoch_end, I think) to loop over the special frames, and send the callback to the Trainer costructor.