u can do that by making a tree from the stack contents and then traversing the tree in either inorder or preorder as you wish
if ab+cd+* is your postfix stack(rightmost being topmost) the tree would be
to make the tree you will need to.....wel im not sure rite now what exactly but i'll get back to you in a day
/ \ / \
a b c d
im sure this cant be too tuf
another way i can think of is starting from the left and parsing the postfix string......
sorry my ideas are a little incomplete rite now...