If you want lower gears, you could:
* replace your current rear derailleur with a 9-speed Shimano MTB RD (XT-M772 is a favorite of mine), which would index perfectly with your 10-speed shifters, but allow you to fit an 11-36 cassette, giving you a granny gear lower than 1:1.

this is great info for me
So If I get the 9speed mtb rear derailleur then I can put on a 11-36cassete and all I will miss 2gear? From 20to18 gear?
Or do I have to do both and change the crankset to?
And again thanks a lot for the info cos I want to do some touring with my sirrus but i need better grannygear