I'll give it a crack. Basically what you would do is set the push pull switch up so that when pulled the neck pickup would be hooked in parallel with what ever else was turned on. when pushed, it would do nothing. This way, when you have the bridge pickup selected, you pull the pot up to activate the neck pickup as well. If you have both the bridge and mid, pulling the pot would give you all three, and if you have just the mid, it would give you neck mid. If you select the neck pickup, or mid and neck, it would do nothing as in this setting, the neck pickup is already in parallel, thus the switch is redundant.
You could do this by running the hot leads coming from your neck pickup through one of the non-common (probably the top pair or bottom pair) lugs on the push pull pot. Then from there to where ever it would normal go on the selector switch.
Then you would wire the matching common lug on the push pull pot to go into your volume pot along with the hot lead from the selector switch.