FIT files that work in the Simulator from GPX files.... again

Hello all,
Does anyone have experience taking GPX files, and converting them to FIT files that are accepted by the Simulator "Playback File" feature ?
If so, please, please, please let me know how you accomplished that, tools used, procedures used.... pitfalls and work-arounds... all of the details.
I really hope that someone out there can provide a solution to this.
Thanks