Modifying parallel port

What is the best way to modify the parallel port setup to include hardware limit switches?
I don't want to take apart and modify the cnc machine control unit which currently just operates the 3 axes motors. So I'm thinking about making an intermediate parallel port box that sits between the PC and the cnc machine control box. This box is connected to the PC and the cnc machine control box by normal parallel port cables. Internally, the box would pass through the signals between the PC and the cnc machine control box. But extra sensor wires from the limit switches would enter this intermediate box and connect to the parallel port going to the PC. Is there anything wrong with this approach?

I'm lazy. It was simpler for me just to stick in another parallel port card, rather than fab a box, or custom cable. (Actually in this case I didn't really add anything, the machine was using a 2 port card and I just plugged in a 3rd cable to the motherboards on-board port that wasn't being used.)

So it looks like I'm on the right track. I'm going to follow the "Home and Limit Switch wiring options" described in section 11.3 of the linuxcnc Stepper Configuration Wizard. No problem there except what do I connect the other side of the pull-up resistor and limit switch ground connections to. Linuxcnc software is evidently aware and need the 0v and 5v lines as they are mentioned in Section 5 (Parallel Port Setup) when describing the "invert" check box. I presume I can just ground the limit switches to the cnc table framework - but I'm guessing that the Pull Up resistor has to go to a 5v supply somewhere - but there is no parallel port pin allocated to 5v - so what do I do? Do I just supply the 5v externally to the pull-up resistor - this could be just a 6v battery supply that would also provide the ground for the circuit. Am I right?

Andy - USB 5v supply - what a great idea -thanks

idealy you don't want to ground any low voltage signals to the machine ,
you'll be far better , using screened cables and bringing them all back to one common point the parallel port
if need be , pick off 5v from the pc power supplys 4 pin molex connector as used for hard drives etc , via a fuse holder
or use a breakout board for the purpose which has opto's built in this will assist in protecting the pc , in the event of any failure .

Thanks cncbasher - good idea. I've ordered a breakout board and will do it that way.

So the breakout board has arrived - a 5axis MACH3 Interface board BL-MACH-V1.1 - and so I began to think how I could use it. I can see how you would attach all the stepper motors, spindle, limit switches, etc. to it. But then I got stuck - surely there is something already like this inside my current cnc machine controller box. So I took the lid off that and apart from a huge toroidal transformer, there are 2 interface boards - one that appears to mainly control the spindle motor speed and the other to which the stepper motors are connected. This board is labelled JP-382A and it has 3 molex-type connectors on of which a 2-pin one is connected to the STOP button and the other two are empty - and from left-hand side are - a 4-pin molex labelled PWM-out and then another 4-pin Molex labelled X Y Z G (at least I think it says G).

So it seems as if I can bring the limit switches input wire to this molex with the limit switch earth going to the G. But do I still need to supply the 5v or is that being supplied by the board? Can't seem to find a user manual for the JP-382A board to check but there is other information on other forums that might help once I've sifted through it all. But any comments by the linuxcnc forum would be welcome.

Sorry - after a quick internet search - the connectors are not Molex but JST-XH (or JST-EH - not sure what the difference is even now).