HPD uses Pro-Voice Encoding which is proprietary. No scanner is made, or probably will ever be made to scan it. The only way you could listen to them is with an actual HPD Radio. Or, if you got a radio like theirs and was able to get it programmed by HPD themself.
I used to listen to HPD back in the 90s when nothing was digital. The state spent plenty money to make sure we can't here them. We can still listen to EMS, lifeguards, and Da Bus.