It was common around the turn of the century in the Midwest to "push up" gable roof barns into gambrels to get more room for the taller haystacks that could be made with the hay track syatems that were becoming popular. I don't know if I have ever seen a wall "extension" done to accomplish the change.

It would seem that adding a post or two under the principle ties would allow for a simple purlin system above to support the gambrel hip. The existing rafters could be used for the "lowers" and new rafters installed above the purlins. With a 5' knee wall there would be plenty of headroom and the savings in the cost of framing, siding, insulating and finishing the inside of the 3' extension plus buying all those trusses would be significant.

Rudy