Without seeing what the geometry looks like there are two options where Rhino 7 does have a bit more available than Rhino 6.
If staying in Rhino 6 make sure to break the model into groups or sections rather than trying to go after the whole model at once. If the mesh is a tri-mesh then it’ll be harder than if it is a QuadMesh as the QuadMesh will be easier to convert to NURBS. So let’s go with the glass is half full and that the mesh is a QuadMesh for which you can then select the mesh and choose to “Convert to NURBS”. Considering that this is a fairly large file you’re going to have to take some time to let the software run. This is of course directly related to the power of your computer. (Make sure to save often). Once converted to NURBS then exporting out as a (.Step) will be possible.
The best option, at the moment, would be to download Rhino 7 (Beta). Once there I’d recommend NOT trying to tackle the whole model at once. Make sure to try, if possible, to place the model to be directly on the origin so that symmetry can be used where possible.
Some of this is going to be some trial and error to see how the results turnout. The new QuadReMesh is what’s going to give the option to give symmetry when converting the mesh to a Sub-D. You’re going to want to select the “preview” button so that you can see what the results will look like before actually confirming. Then it’s possible to convert the model from Sub-D to a NURBS.
It’s not pretty but it’s the way to get the job done for now.