User material subroutine for Johnson Cook Model in LS DYNA

I need to edit user subroutine for existing Johnson Cook model (MAT_015) in LS DYNA. I have downloaded object files. However, I could not find any subroutine file for MAT_015. I need to edit the model and add 2 more parameters to increase its accuracy. Creating it from scratch is really a lengthy task. Can anyone help me in this regard? Thank you.