You have a couple options for increasing the output range of the DAC chip.
You could use an op-amp to amplify the output voltage. I found this page which has some useful information on making a single supply op-amp circuit.
The second option, if you don't mind having the ADC DAC Pi seperate from the Raspberry Pi is to use a buffer on the SPI bus and run the ADC DAC Pi at 5V. On the Expander Pi we use a 74HC4050 buffer powered from the 3.3V supply on the Raspberry Pi. The inputs on the 74HC4050 are tolerant of voltages up to 15V so you can power the ADC DAC Pi from the 5V rail on the Pi and the 74HC4050 will act as a level shifter for the SPI bus. The Expander Pi schematic shows which pins need to be connected on the 74HC4050 to the SPI pins.