You would still have to open it up to parallel the small battery with the built-in charger. Paralleling SLAs is ok as long as they use the same internal chemistry, or ones made by the same maker. I would not parallel an AGM with a VRLA or with a flooded-cell Lead Acid battery.
If it is just to trick the PortaPak into starting up, you could likely use a momentary pushbutton, a diode, and a 9V alkaline cell. The diode would prevent back-feeding the 9V battery once the Pack starts up...