I have the WK2 oriental arranger (midi module) right here in front of me and it definitely does have the mixer lock function. Since the keyboard you're using is also an oriental, there's a good chance that this has it too (the Oriental designers often added extra features that were not on the "western" models).
You should find it in the GENERAL edit menu, after Microtuning. Unfortunately my WK2 Oriental keyboard is packed away somewhere hard to reach so you'll have to confirm this for yourself the next time you get your hands on the instrument. The edit menus are accessed by moving the cursor along the left side of the screen. Then use the right cursor button to page through the contents of each menu.