I'm afraid it's something only good photo-shopping can do since you'd have to fit all the realistic proportions into the FH model proportions...
It might help to copy and paste different sections of the dog's head and fit them into place first? Muzzle, head, nose, eyes mainly. With the FH model though you're not gonna get that lower lip flap since the FH model has a wolf muzzle, completely different to the breed you have there so I imagine you might have to raise the muzzle in the image to the UV map line and create a jaw or something!
Using the fix or patch tool should help too! I think it would look like a band-aid? Using that to create fur/skin in empty areas would help a lot. Maybe look up some tutorials on how to do that! But unfortunately there's not much else to say or offer advice with other than asking a map maker who has had experience with doing similar things.