I've never heard of anything for removing roots other than a professional. Now, Rid-X works for dissolving grease, paper, etc. In your tank, but it won't tackle tree roots.
Once tree roots have infiltrated your line, there is no treatment that will totally remove them. There is however foaming treatment that will temporarily dissolve the roots, and also take away from the biological properties of your septic system. Zero additives should be added to any septic lines or city sewer for that matter. These chemicals eventually end up in the ground and on to the water table. In the case that you have roots in the line, you should first find where they are with a fiber optic sewer camera, locate the exact position and replace the section of piping as needed.
Don't think adding rootx in the septic tank is going to get to the field drain. Expose the field drain after the septic tank and add the rootx to the field drain. This would be more effective and doesn't kill the septic tank process.