First you would have to export the nif model as an 'obj' model, through a nif model viewer, i like to use a program called NifSkope, google it.
Load up your model through NifSkope and export it as an obj.
Open up your Obj model with a program called Metasequoia
Do the whole modeling process, name parts, scale, reposition and such.
Save your new model to a mqo file
Use Yukoshoo's mqo-pmg tools for conversion from mqo to pmg (drag the framework and the model onto the converter)
Load up your PMG Editor, check for errors, load up PMG Viewer, check for errors.
Place your new pmg model into the data folder path
Good Luck, I use this method for creating models that i have previously made for oblivion or models that i have extracted.
ComplicatioN
Mabinogi Status