OK. Sorry about that, but at least we are on the same page now!
Here is what I would do now...
1.) "MENU" to: "wrench-1" and set "Menu start" to "Previous"
2.) "MENU" to: "camera-3" and set "White Balance" to "_ SET" and follow the prompts (which you are already doing).
If you feel that the custom set needs redone...
3.) Because of (1) above ... "MENU" / rear wheel "CENTER" / rear wheel "DOWN" ...takes you fairly quickly back to resetting the custom WB.
++++
Now, with all that said ... I think I know what you are really looking for. Accessing this w/o diving into that routine. The answer is YES and NO.
You CAN NOT initiate a shot to SET the WB from the Fn menu, but you CAN do one of two things, manually.
From my previous post, I will continue.
1.) Press Fn
2.) L or R click rear wheel until the ORANGE bar is under the WB footer
3.) Rotate the rear wheel (or front ring) until the WB setting is as desired (which would include a custom stored option as well as all of the defaults)
4.) Press the "CENTER" button of the rear wheel to select
OPTIONS
5.) To select the custom WB set from the MENU procedure, ORANGE bar under "Custom." Now, rear wheel click DOWN. You CAN NOT change the K value, but you CAN change the A-B and G-M values!
6.) For a completely new custom WB set from the Fn key ... ORANGE bar under "C.Temp./Filter" with the K icon. Now, rear wheel click DOWN. You CAN change both the K AND the A-B and G-M values!