Mine is the FDE but like you I want to know if I can add a manual safety to it. I just sent an email to Springfield's support page and will let you know what I find out.
I want to know if a safety can be added to the Echelon that I currently own? I also own a Sig M18 that I was able to buy a manual safety for and then added it to the M18. So that would be awesome if I could do the same with the Echelon.