I've had a quick browse through, and their fee calculation seems to be done by estimating the fee based on the likely number of blocks you'll need to wait until it gets confirmed ie. LowPrio = 20 blocks, Economic = 10 blocks, Normal = 3 blocks, Priority = 1 block.
How this gets converted into an actual bitcoin amount I'm not 100% sure of at the moment... some magic involving their API I think (my java skills are a bit rusty)... whereby the size of your transaction is calculated and current "market" fee Per Kb is used or something...
However, theoretically, you could probably just add a text box or slider or whatever into the UI to set the value you want, and then just modify all the calls to createUnsignedTransaction() and pass in whatever value for the Miner Fee that you liked...
Theoretically...
