Sorry i could'nt get u about multi rom..................... I m not able to flash it in multirom as secoundry
U cant use this as secondary. U must install it in primary
1.backup all portion of primary.
2.Wipe data,delvik,cache,system in primary.
3Mount system,data,firmwire.
4flash 3.21/3.2.6 firmwire zip.
5.flash aicp rom then gapps 7(64bit) micro/nano
7.Boot aicp.run the rom then reboot to recovery
6.Add rom ,select from backup use primary rom backup.(if u want to work primary rom again but this time it will be in secondary.)