In order for the pin to retain the voltage, you will need to add capacitors to the pins. You can do a simple TopDesign configuration like shown below for your design. Make sure you use VDAC's output buffer configuration as "Buffered to pin". This enables OpAmp buffer on the VDAC output path and provides fast enough charge source for the caps on the external pin. Using the below config and external cap, you can simply change the pin you want VDAC to drive using the AMUX API "AMux_1_Select" or "AMux_1_FastSelect".
Let me know if this helps.
Meenakshi Sundaram R