*.dat is not a file format, it is a generic extension used by many programs to mean many different things.
What kind of *.dat file do you want to convert the *.hrm into? What is your destination program / application?
The *.hrm file format is extremely simple. What questions do you have about it? The file is pretty much human readable.
The columns are just x,y,z -- each body is made up of a bunch of points in a cross section. Every cross section has the same number of points. Each body starts by telling you how many cross sections and how many points per cross section.
Rob