Whether or not you can add wing extensions or winglets I cannot say -
but - I believe people on this thread are being unfair to the PW-5.
We have several in the club (private and club owned) and they are much
loved. They are a great stepping stone ship for new pilots to move
from trainers on their way to slick modern long wings. Inexpensive,
benign, a reasonable XC ship, very easy to rig, and, from what I can
see, strong. Don't be a snob.