Hello there.
If your models are in OBJ format, then you're in luck.
Milkshape 3D can natively import OBJs and there's a BC NIF i/o plugin by Assimsoft.
So you can import the OBJs in Milkshape and then export them as NIFs.
A few things to consider though:
Each submesh can only have one material.
Texture files need to be in TGA. Colors are in the RGB-channels and glow/emission in the Alpha-channel.
The Specular-map is in its own TGA file as the usual grayscale.
You can pretty much name your texture file how you want, but the optimal practice would be to name the texture files like this: [mesh-name]_glow.tga and [mesh-name]_specular.tga.
Also, please make sure the texture files are properly assigned to each submesh. :-)
(Unfortunately, there's currently no BC NIF i/o plugin for modern 3D modelling software like Blender.)