It seems you've been trying to sort this for over a year now. A quick blat around the forums suggests there are others in the same boat and no one has an easy answer. Its a proprietary control system and very few 'proper' CNC software recognises HPGL as a useful command language as it proprietary and limiting. DPML is even less common. I think you're on a hiding to nothing here...

I would suggest the time has come to cut your losses, and replace the SEI controller with a PC running MACH3. How much of the electronics you can use remains to be seen, however steppers etc are reusable. With some judicious ebaying it need not be too expensive. At least then you'll have full and complete control over the chain of events and the end result.