The results of a full simulation of an endcap Time-of-Flight detector upgrade based on multigap resistive plate chambers for the BESIII experiment are presented. The simulation and reconstruction software is based on Geant4 and has been implemented i
nto the BESIII Offline Software System. The results of the simulations are compared with beam test results and it is shown that a total time resolution $sigma$ of about 80 ps can be achieved allowing for a pion and kaon separation up to momenta of 1.4 GeV/c at a 95% confidence level.
