I don't think this is possible as when you load a new setting, all the old settings will vanish!
However, if you want Pan set to zero and All sounds to Main then Option-Click on the Pan and Output routing section for each assigned sound.
Another option is to set up the routing the way you like it and load a kit a different way:
1. Make your routing settings (save your setting too just to keep a copy).
2. Click on the Import button (top of interface) and select the drum kit you wish to load.
3. Click on the downward arrow next to the Import button. A list of the sounds from that instrument are displayed.
4. Click on a name to preview it. Drag it over an existing name in the routing area to load it into your current kit and setting.
Notice the routing stays intact :)