Yes, you can control the TRIM output directly by writing values to the DAC register using the I2C port. To do this you must set the TRIM output in the I2C control mode for profile 0. This can be done with the POWR1220AT8 or the POWR6AT6 device.
The simple method to do this is to open the design in PAC-Designer and use the "Edit Symbol" option under the Edit menu. Then choose the trim number you will be using and a dialog box comes up where you can select the I2C control for profile 0 (the bottom entry) and the DAC output range. The DAC output range value will be the value which is output when the DAC code is 0.