If you are going to add hexokinase, you should also think about adding some excess glucose too. Cellular [glucose] is approximately 5 mM, and [ATP] will vary depending on cell type and energy state. As per the SIGMA website for Hexokinase (H4502), One unit will phosphorylate 1.0 μmole of D-glucose per min at pH 7.6 at 25 °C. Depending upon how you prepare your hexokinase, I would add ~ 60 units of Hexokinase (with a final concentration of 5 mM glucose) and leave the reaction to run for 1.5 hours. An alternative method would be to add Mycokinase and an excess of AMP (which reacts with ATP) and will form 2X ADP molecules and is very rapid.