Hi,
Welcome to the forum.
Usually, machines that use HPGL are driven by a serial connection and Mach3, essentially, uses the computer's parallel port for it's machine communication.
Obviously it depends on many factors but, basically, if your Precix is driven by Bipolar Stepper Motors then it is pretty easy and straightforward to replace the existing drive electronics with something like the Gecko 540 controller
http://www.geckodrive.com/g540-p-39.htmlThere is a bit of skill involved but it is not rocket science.
If the Precix uses servos instead of steppers then it is still doable but slightly more complicated and costly.
There are many, here on the forum, who have undertaken a similar task, albeit a different machine, who will be willing to help with any problems you encounter if you decide to take on the task.
Good fortune.
Tweakie.