If you want to give the truncated access to the , the only change that needs to be made is extending the southbound platform at Botanic Garden. The would then run on a single track between Franklin Avenue and Botanic Garden.
Of course, the main problem is that neither Botanic Garden nor the connection to the are ADA accessible. So either you would still need to run shuttle buses, or you'd have to figure out how to make the complex accessible. I think you could get away with installing 2 ramps and 2 elevators, but even that would significantly add to the project's cost.