It is possible to test mapped network drives using either the standard disk test (by selecting the drive from preferences) or from the advanced disk test.
However, on Windows Vista and later there is an issue relating to the User Access Control (UAC) and mapped network drives. Mapped network drives are not visible when running as elevated administrator, which PerformanceTest is.
There is a workaround available that makes network drives available with elevated admin, and by extension available for testing in PT.
A more complete description of this issue and the workaround as explained by Microsoft can be found here http://support.microsoft.com/kb/937624