Question about combining pieces into one nif

Hello,

Is it possible to combine seperate NIFs with HDT physics into one NIF? For example, I wanted to combine the upper and lower nifs from Christine Sexy Sorceress into one NIF so I can use that NIF as a replacer for the archmage robes. I went through BS, imported the lower nif to the upper nif, conformed the lower nif to the body, changed the partition for the lower to 32 and saved. I equip the archmage robe and the lower part is static. What step am I missing? Do I need to do anything with the virtural parts?

Thanks in advance for any information! 🙂