I created Shiai items to be items and not currency because 1. It'd be a butt to implement that, and 2. hampa didn't like the idea of it being a currency.
Shiai should, and hopefully will remain an item that you can redeem for prizes.
There are ways to implement a converter between Items and TC; which of the easiest would be to set up a account running a trading script which is checked like once a week... (Send the account some tokens and receive the TC equivalent per-say the current trade-rate.)
Currency is stored as a "number", Shiai are items instead.
No way to implement an automated exchange for shiai items now until that's fixed.
If it's changed, that'd be probably the first thing to add.