MGT interface belongs to MGT zone. MGT zone is a functional zone so MGT interace cannot have MIPs.
Before ScreenOS 6.2, MGT zone is hard-coded to MGT interface.
From ScreenOS 6.2, you can assign a security zone to MGT interface and therefore use MIPs
nsisg2000-> set int mgt zone untrust
nsisg2000-> set interface mgt ip 172.19.47.61/23
nsisg2000-> set interface mgt mip 172.19.50.120 host 1.1.1.1