This usually set by an option in each aircraft, so using planemaker might be a start. But, generally, if not addressed by the acf file for an aircraft, there is a setting in your preference file that can address all barometric and temps that are not set up for each aircraft. Make sure XPLANE or PLANEMAKER is not running.
In your XPLANE/OUTPUT directory there is a PREFERENCES directory that has all of the settings you have set while using the simulator, whether it be for user profile, joystick and keyboard settings, etc. CAUTION, make a copy of these files BEFORE you even open them because even the smallest of changes can effect performance. These are ascii text files, only edit with a text editor that does not save in rich text.
In the xplane.prf file find the two lines:
0 = off 1 = on
Change the 0 to a 1 and save the file do not change the file type or name.
This will change all temps and bars to metric, if not addressed by a specific piece of equipment, i.e., the G1000 has a setting for the barometric units for either in or hpa. Other aircraft may have similar settings so if you want to change them, try looking in to the plug-in options or again planemaker. Payware aircraft and plugins may completely override or ignore these preferences.
Hope that helps. ;)