I don't have an IBM, so I can't comment on that exact unit, but I did something similar to a Dell docking station.
Mine has a switch with two pins that are connected when you press the switch, so I just wired an external switch in parallel. The switch I used is a simple single-pole momentary contact pushbutton switch that is normally open (when you aren't pushing it down). To make it easier to disconnect things, I mounted a small two-wire audio jack on the docking station and wired between the switch pins and the jack. Then I made a cable with my auxiliary switch on one end and a mating plug. That way, the docking station is easier to remove if needed. I mounted the jack to a hole I drilled in the plastic case.
(Be real careful if you start drilling holes, to make sure you don't damage something or drill where your jack won't fit!)
Some general comments...
If you get different action by pressing on the switch for a longer time, that tells me that it must be a momentary contact switch. In other words, the switch contacts change to either opened or closed only when you press on the switch and don't stay in those positions when you take your finger off.
I would think that it probably is a switch that makes contact when pressed instead of the other way around. That is a pretty common way of doing things.
Are you sure all the pins on the switch are actually used? 5 sounds like a lot. Some may only be there for mechanical strength. The following pretty much assumes that only two of the switch pins are actally used...
If you have a multi-meter, you can measure the voltage on the pins and see what readings you get when you are and aren't pushing on it. Look for pins where the voltage readings change from not pushed to pushed. There should be one always at ground or at some voltage too. I'd expect the pin with the changing voltage to have the same voltage as the other pin when the switch is closed. That will give you clues as to what pins are connected when the switch is pushed.
If you have a meter that can measure resistance, you should find that one pair of pins will have high resistance in one switch position and low resistance in the other. I'd probably use the resistance check method first WITH THE POWER OFF! Hopefully, you'll find two pins that have high resistance (open) and then low resistance (closed) when you push the switch. If that is the case, then you get a single-pole momentary contact switch (contacts closed when pressed) and connect one pin of your external switch to each of the pins on the docking station switch you identified above, putting the switches in parallel.
Now, if it isn't a simple switch, then it gets more complicated, and thats beyond the scope of this reply.