Well, first you should play GP4 with GPxPatch which allows you to choose different tracks for any slot.
1- Before, you should check the properties of the track using CMagic tool. There is a button that allows you to see the so Called GP4info values. There you must create a property called Prefix, and assign the value "hockenheim2001".
2- Then load the track with GPxPatch (GPxTrack tab). This property (Prefix) will allow you to put this track in any slot. But do not rename the wads, I'm not sure but I think it doesn't work.
It is not difficult, but it requires some basic knowledge of the GP4 available tools (GPxPatch and CMagic 102 mainly).
Cheers