Yes you can if you no other choice as a temporary solution and if it is just for indication or trip purposes. Yes it will measure the 0 to 6Bar but, the measurement will be very insensitive to small changes in the process so it will be a very bad idea if you need it to do control with. Your control will become very unstable since it will get untrue/more or less readings from this transmitter and the PID controller will be overcompensating all the time.
Keep in mind the process will most probably not be at 6bar all the time so you are probably looking at a average of about 3 to 4 bar where the process will be controlled.
If you have to use the transmitter keep in mind that it will basically be just a very course estimated value that will be displayed by the transmitter so it is obviously not the right way to do it.
As you most probably know, you need to use a transmitter with a max capability as close to the span calibration as possible. In this case anything from a 0 to 7, up to 15Bar transmitter will be fine to use for accurate control.
The further you move away from the span with your transmitter selection the less sensitive and inaccurate the transmitter will become so just consider where you are with the 200Bar transmitter measuring only 3 to 4 Bar.
