Prepare the buffer, then add EDTA (preferably adjusted to pH 7) and dithiothreitol from concentrated stock solutions. Dithiothreitol will become oxidized in dilute solution in equilibrium with air, so it should be added just before the solution is to be used.

Commercially prepared 0.5 M EDTA stock solution is typically at pH ~8, so it will very slightly increase the pH. A 1 M dithiothreitol stock solution can easily be prepared.

If you don't want the phosphate buffer to be slightly diluted by the two additions (by about 5% if you use a 0.5 M EDTA stock solution and a 1 M DTT stock solution), start with a more concentrated phosphate buffer solution.