Can I lace a XTR rear hub with thirty two spokes from a Valiant asymmetrical rim to a thirty two hole Magic regular rim?
Yes. The hub doesn't really care whether its spokes are laced to an asymmetric rim or not.
However, you may not be able to reuse the spokes--regardless of whether you're switching between an asymmetric rim and a "straight" rim, the spoke lengths may well not be close enough because of different rim depths. You'll need to check a spoke-length calculator.