The biggest problem will be with your frame. A XC frame isn't designed to handle the repeated abuse inflicted upon it by dirt jumping. Can you do it? Sure! Will it last? NO!
Plus, going from a single crown XC fork to a triple clamp Jr. T is going to make the bike handle like a wet noodle. Plus, one bad landing and your headtube will snap right off the frame!
Changing the cranks should be fine, but I would invest in a new frame before I bought some new cranks!
Actually, if cash is limited, keep your XC bike as is, and just buy a new complete DJ bike. Almost every critical component is far too different and you'll go broke trying to convert!
L8R
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"Life is not a journey to the grave with the intention of arriving safely in a pretty and well preserved body, but rather to skid in broadside, thoroughly used up, totally worn out, and loudly proclaiming, "WOW, What a Ride!" - unknown
"Your Bike Sucks" - Sky Yaeger