I was going to direct you to a post I made long ago helping someone who was unable to split a stack of items (shift-click on the stack didn’t work) because “SOCKETITEM” is a modified click just like “SPLITSTACK” is… but /dump GetModifiedClick("SOCKETITEM") returns an empty result for me yet I can still open the socket window, so I am not sure the directions I give in that thread would help you (not sure if Blizzard is doing something different or if I have some addon playing middle man and changing the behavior to not use the modified click settings).
I guess it doesn’t hurt to try though. 