Disable index shifting to create friction shifting possible?
Typically not -- a few index shifters (like my Shimano 105SC 7 speed ones) have this option, but its primarily downtube and bar end ones for &lt;=8 speeds. I don't think there exist any other shifters that do this other than possibly some grip shifts since they pretty much are letting pawls catch. 

However, you can buy friction shifters pretty cheaply these days from companies like Sunrace.