Could I specify PATH for MPI slave node in the job script file ?

You can specify for MPI slave node almost all environment value by "#PBS -v option". But you can not specify some environment value on NQSII. Path is one of those. Please see the following manual, the end of 1.16.qsub(1).
NQSII User's Guide

You can specify PATH for slave node by the MPI runtime option.