How to use coin cell CR2032 in parallel with a power supply?
If you regulate the supply to 3.0V then it should be OK. A coin cell that is nearly flat will get charged slightly (until its voltage rises to match the supply) but that shouldn't hurt it. Voltages higher than 3.0V might damage the cell. If you can't hold the voltage down to 3.0V then you will need some way to isolate the battery. Here are a few ideas:-


Put a high current Schottky diode in series with the coin cell, which should drop less than 0.2V at low current.   
Install a jumper or switch to disconnect the battery when using external power. If the voltage difference is small then it should be OK to leave the battery connected for a few seconds during the transition, otherwise you could include a large capacitor to hold the voltage up between switching the battery out and applying external power.  
Disconnect the cell automatically using a power jack which has a built-in battery disconnect switch. 
Use a FET to switch the coin cell in when external power is not present. Unlike a diode this has minimal voltage drop when on battery power.