landscape - How to mount a circular ND filter in front of the ND graduated filter?

Recently I'm into landscape photography and trying to build a convenient and practical setup.

I have the Cokin Z-Pro filter holder, Lee ND Grad filters and as solid Hoya ND16 filter. However, to set everything up takes ages. First, I have to install the Hoya solid ND filter. Then, screw the filter holder adapter on, mount the holder and then put in the ND grad filter. All this needs to be done before focusing and composing as once you mount the Hoya ND filter you won't see anything through the viewfinder.

So my question is this. Can I mount the solid ND filter in front to the ND grad filter? That way I can focus and compose with the ND grad filter on and then add the solid ND filter at the end.

I saw a few videos on YouTube where people just slided the solid ND filter / polariser in front of the ND grad.