This will all depend on what kind of servo drives you have on the mill. It is likely they will be analogue only amplifiers and the feedback would have gone back to the control, if that is the case then you have a few options, external motion controller or get new drives.
The controllers that come to mind are the
DSPMC
http://www.vitalsystem.com/web/motion/dspmc.php#Mach3KFlop
http://dynomotion.com/Ajax
http://www.ajaxcnc.com/Galil
http://www.galilmc.com/I have no experience with any of them so best do some searches
Depending on size and type of servos there are lots of drives available, see here
http://www.machsupport.com/forum/index.php/topic,12233.0.html for a write up Simpson did on various ones, looks like the Dugongs are nice drives.
If however your drives can accept Step/Dir signals then you could connect direct to the parallel port but I would advise to go with a smoothstepper, I use them on all my machines and love them.
So basically you need more info before more concrete advice can be given.
Hood