This cork butt addition will depend on the type and design of the reel seat on your rod. If the reel seat foot has no butt plate, then it may not be possible without replacing the entire reel seat.
It can be done on a downlocking reel seat having a butt plate. To do the job correctly you will have to remove the current butt from the reel seat. Heat from a heat gun should do the trick to loosen the glued on butt plate.
One option for attaching a fighting butt is to purchase a cork fighting butt from one of our sponsors or a place like Cabelas, install it onto an old piece of rod blank, then glue that assembly into the butt end of your 6wt.
I did this same thing to the first fly rod I had ever built when I started building/repairing fishing rods and the butt addition turned out beautifully. It certainly made me plan better when it came to building rods.
David Parker
Guild Certified Professional Rod Builder