That is a great idea on how to adapt the walkers we use . One of the wheelchairs I use is called the Invacare AT'm. I have a rehab type chair(Which my walker fits to the back of, for easy removal when I am in the position to do a little walking) that I use with my disability van.
But the AT'M is a supplemental chair, like a regular fold up wheelchair with a small detachable battery with a motor on it .The chair easily folds and fits in a in to a car trunk and very lightweight, so can be used without having to use my handicap van.

Further down the line a optional small motor could be added to your invention,(similar to that on the AT'M) making it truly a one size fits all assistive device. Many people regardless whether they have PD or other physical disabilities could revert from stand alone walking,assisted walking whilst seated or if needed motorized assist with your invention. An all in one device like this would do away with the need for chair and walker and would be great whilst out and one needs further assistance to walk as the day goes on.

well done!