Yes, was considering this in the past, alas,
GW deleting ETB aggressor pack really put a dent in these plans
If you want silhouette to read even more terminator like, you can replace aggressor backpack with regular power armor one, and cut off propulsion vents. This will help them to maintain same square top shape. As for helmets,
IIRC the problem is that terminator ones glue at the back, while aggressor ones have necks. You'd either need to get creative with gluing or add a neck to terminator helmet.
I'd show the pictures of my conversion but it's unfinished and somewhere in a drawer 'to do later' so can't help right now, sorry.