Is it possible to use VM converter to import a local machine that has its disks mirrored?
Yes, it's possible if the mirror is hardware level (RAID 1). In fact, converter must see one disk.
Hardware-level mirroring through RAID is fine. In some cases, software-level mirroring (e.g. through Windows Server 2003) may cause Converter to fail and the resolution is to dismantle the mirror prior to running Converter.
software based = no
hardware based = yes
however, if it's NT4 = any fault tollerance will fail (software or hardware, any raid config)