Granted, as on most aircraft it is possible to do. Part of the issue may be the SAS/AP DCPL switch being on the cyclic ( At the top, furtherest Left) and on other aircraft the SAS disengagement is somewhere else. You probably wouldn't want to do that in IMC. This will dump EVERYTHING - Yaw SAS, P/R SAS and Pitch Damper and of course the AP will go off in sympathy as it needs it's friends. APMD DCPL is the correct one to use which is right down the bottom on the left. This will cancel all upper AUTOPILOT MODES.
The AUTOPILOT on the 135 can be used ALL the time even in the hover and is normal procedure. The control functions provide much improvement over the standard Y P/R and P. If you wish to turn the AP OFF you should use the switch on the APMS.
One EC135 has come to grief in the UK because the AP did exactly what it was told and the operator was not familiar with the system.