You can use the 4x5 sheet film, and the 4x5 packfilm.
For the sheet film you'll need the Polaroid 454 holder.
For the packfilm you'll need the Polaroid 550 or fuji PA-45 holder.
I've the Polaroid 550, and it doesn't fit exactly. To get a better fit, you would have to remove a small part from the Harman Titan. I recently did it myself, and it's a bit of work, but has no negative effect on the fit of other backs.
It should also be possible to fit a holder for the smaller pack films ( type 100 ). But most of these have the dark-slide on the wrong side.
Because all polaroid holders are thicker than the normal DDS holders, you'll have to remover the spring thingies on the back of the Harman Titan. I haven't found any way to mount a spring-back. So you'll need some tape or rubber bands to hold the backs in place.