Yaakov,
I will admit that I am not 100% sure what your are attempting to accomplish, however I will try to provide information the best that I can.
First off, I think you really need to evaluate if S curve motion planning is something that you absolutely need, if it is just a nice feature, or just something you’re interested in but don’t need. Mach 3/4 is currently not offering S curve motion planning and I wouldn’t bank on it being changed anytime soon (although I don’t work for NFS, so I don’t know what they are actively working on
). If you absolutely need higher order motion planning, there are many products on the market that can offer this, although I do not know of any at the price point of Mach 4. If it isn’t absolutely necessary for you then I wouldn’t try to go down the rabbit hole. Maybe there are other, possibly more elegant and cost effective, solutions for your needs.
With all that being said, there is no reason that you can’t manage your Gcode to adjust feedrates to simulate the motion generated by higher order motion planners. Many fancy CAM suites offer features like feedrate reduction into corners and high non-rapid feedrates when the software knows you are cutting air. You might be able to set up your CAM to segment all your Gcode with variable feedrates. But again, why? That seems like a lot of work and your issues might be more easily solved in other ways.
My opinion is to leave the purchased software packages to work as they are intended to.
Can you elaborate on what is driving your interest in S curve motion?