Why not just add 'extention of stay'? Surely it'll just add confusion to people by selecting the base of the extention, as if there isn't enough already over visas and extentions!
Probably wont be an issue for Thai Immigration but it's boarding your flight that you'll have problems. The vast majority of long haul flights/carriers require you to have return or onward travel within the allowed entry to your final destination, ie in your case 60 days.
Then you have a fraudulently obtained visa/extention. Forever looking over your shoulder, if the shit hits the fan it'll be all over you not the agent!