Is it possible to get more suspension travel from the rear suspension?
Anything is possible if $$ isn't an object. You would need to have a longer stroke shock made and valved accordingly. There might be other issues that would come along with it such as the possibility of a chain idler to keep the chain slack at a minimum when the shock is at it's longest part of the travel.
Simply making a ride height adjuster would only suffice for making the bike sit taller and gaining ground clearance, but it wouldn't add to suspension travel.