18" Rear wheel on XT 660 Z Tenere
 

I've got an 18" wheel from a YZ 250, can I use the spokes and rim and lace it onto the XTZ hub?

Does anything need to be modified?

Reason is so I can fit Tubliss conversion.

That seems like an odd reason to go to an 18" rim....
Do both have the same number of spokes? I think you should ask a wheel specialist how to go about it.

Or check this out, as an alternative to Tubeliss: http://www.bartubeless.it/en/

Both are 36 spoke.

It's not just for Tubliss although that is the main reason, Tyre choice is much better for an 18" wheel.