Hi,
I don't think it is possible to put the K60 TWR board into MII mode. Based on the schematic you can reroute a 25MHz clock to it using jumper 6 but the needed MII signal lines from the K60 just get routed to the IRQ jumpers on the SER board (take a look at the K60 TWR schematic and follow the PTA24+ lines, which are the MII signals). The SER board can be set for MII and can be used with Coldfire processor (this information can be found in the K60 tower uTasker demo PDF).
I am not sure why the K60 TWR was configured this way. It might have to do with wanting to get more access to different peripherals.
Hope that helps.
(Although if you do find a way, please let me know!)