What do you mean by a "keeping room"? I wasn't familiar with the term, so I googled it. The results I found tell me that in today's terms, a great room is the same thing as the keeping room.

This plan has a great room, so I'm not sure what you're looking for.

A keeping room is an additional great room/den which is directly off the kitchen. Here is a plan with a keeping room just as reference:

http://www.frankbetz.com/homeplans/plan_info.html?pn=3734&vw=3

I had to look up what a keeping room was too. I love Frank Betz plans, but a keeping room is not a common thing where I live.

I don't know anything about cost wise to add it to the plan, but (if you haven't already) it might be worth your time to go through more of the Frank Betz home plans. A lot of them include a keeping room, so maybe you could find a plan that still has everything you like about the Kensington with the added keeping room.