hmm. well, i can tell u one way but i don't tihnk it will be "that" great but its worth a try. in photoshop, on a new canvas, with the default colors b/w, go to filter>render>clouds, and then filter>render>difference clouds. then use the brush or gradient tool, (make sure ur foreground color is black)which ever one ur comfortable with, brush the part where u don't want the clouds to be, make it so like it looks real. then save it as a jpeg. after tht, open after effects, import ur clip there and also import the fog cloud thing. put the fog cloud on top of the clip, and set the mode of the fog layer to screen, now all u have to do is animate it, also put more fog layers on top so it looks better. i hope this helped u, i know u dont' have after effects, but u can download a trial version until u purchase it.