Your post processor for your CAM should be able to be altered to add the G64, you can also change things on the General Config page so that the default for Mach is Constant Velocity. The problem with relying solely on that however is it can be changed by the code so best to get your CAM to do it if possible.
G91.1 is Incremental for IJK mode, you want to keep that in your code as well, no need to remove it, just have the G64 on the line after it.
Hood