Since the question is about, how to convince the other person, you will need to have something your supervisor wants. If they want you as a researcher, they will hire you as a researcher.

Others have commented about how grant funds can only be used for stated purposes, so you are definitely going to be an outlier if they hire you as a researcher instead of a standard PhD student.

You should consider options, like if you need to work for one year to obtain permanent residence, find a job outside the University, and if you get an offer, you can renegotiate with your supervisor about being hired as a RA, in this case you would get a work contract anyway, either with your supervisor or outside the University.

Having options is always better than not having them.

Since the question is about how to convince the other person, you will need to have something your supervisor wants. If they want you as a researcher, they will hire you as a researcher.

Others have commented about how grant funds can only be used for stated purposes, so you are definitely going to be an outlier if they hire you as a researcher instead of a standard PhD student.

You should consider options, like if you need to work for one year to obtain permanent residence, find a job outside the University, and if you get an offer, you can renegotiate with your supervisor about being hired as a RA, in this case you would get a work contract anyway, either with your supervisor or outside the University.

Having options is always better than not having them.