You can do this already, using scripting. Just have like a "Combining Page", and when the player goes there create links based on which combination could be made with the items they have. Then when they click the link, the combination gets made (old items are removed and the new one is given to them), and then the link re-directs back to the combination page so they can combine more items if they want.
Or, you could use the global page script to always check for possible combinations, and automatically do it if the player has the right parts.
