I've had a look at the model and sure enough, the motor is part of the PLL, in between the low-passed error signal and the VCO. I see no rhyme or reason for this but again, that may be my ignorance.
The transfer function you're talking about - what is the input and output? Judging by the function, is sounds like you're modelling the motor as an RL circuit, setting voltage as input and current as output? You then stick that in the middle of the PLL, so the input voltage becomes the low-passed error signal and the output current becomes the input to the VCO. What makes you think that will work?
As already suggested, the only way I can think of to actually make this work is to get some feedback from the motor shaft. Hopefully I'm wrong and someone knows what you're trying to do, but it's not looking promising!