DPBS is Dulbecco's PBS and also known as CMF-PBS (Calcium-Magnesium free) PBS. I can imagine your cells will anyway not enjoy to stay too long in DPBS alone. 5% CO2 is used to maintain the hydrogencarbonate levels (buffer) high, cell metabolites usually are acid and hence your cell-medium (pH indicator usually is phenol-red) turns yellow. In my opinion no need to add addtional NaHCO3 if you have them in an incubator with 5% CO2 and anyway in PBS their metabolism is slowed down. Check if your cells are viable after 2h in DPBS. Good luck