Unfortunately, there's no room in the 100 for those modules beyond 2MB. Special (read: extra expensive) '100-specific modules are needed. They are hard to find, and costly when found.
In desperation, I've made my own. You can desolder the chips from a 140's expansion card, e.g., and stack them atop the chips in a 2MB module. Solder all the pins together except for the enable lines, then separately wire the latter to the appropriate card pins.

Click to expand...

So, this card would not fit in the PB100? It's actually not a bad price.

http://www.welovemacs.com/po146mb.html

It appears as though it would extend over the top of the daughter card. There's certainly enough room between the top of the daughter card and keyboard (as there is a spacer there already) and if necessary, I wonder if one could fabricate an extension to raise the RAM card high enough to sit on top of the daughter card if it won't. I've done a similar thing with a spare LC PDS connector and card in order to use a PDS card with a 575 & PPC upgrade which otherwise blocks the slot.

I misread your solution which indicates I actually need the PB 100 card in order to add RAM to it. Perhaps I should pick up one of these RAM cards in anticipation of the say I find a 2MB 100 card ...