You cannot upgrade the indexing on Shimano bar-end shifters unfortunately. However, if you just use it in friction mode, then no problem. A friction-mode shifter can deal with just about any derailer and cassette that you care to throw at it
I for one use SunTour bar-end shifters with Shimano LX derailer + cassette on my touring bike.
I'm not sure about the capacity of your particular derailer, but it's quite unlikely to be a problem to go to a slightly larger cog or larger chainring, as the Deore XT is about the widest-ranging derailer available. What is your current setup, for comparison?