The best thing you can do is to ask for the Merrill Lynch Index fund to be added. I am not that familiar with Paychex's product to tell if there would be an additional cost. What I can tell you, in theory there shouldn't be. Merrill pays Paychex the same as Salmon Smith Barney to offer their funds in Paychex's program. That is why these funds have higher expenses than Vanguard, etc. Paychex gets a percentage from Merrill, Salomon and Fidelity Advisor for offering their funds. Its usually pretty minimal something like 0.25%.
Ask your employer to call Paychex and ask about adding the Merrill S&P 500 Index fund.
Paychex pushed their clients to use their 401k by making it difficult to use any one else. If you use another 401k provider, the payroll information has to be transmitted to the 401k provider. Paychex would charge extra fees for this if their 401k service was not used. Not all Paychex customers are affected by this. Each Paychex office is individually owned and operated, (Franchised) so each owner was free to push the 401k product at their own level. Some did not impose the extra fees, and just simply made their clients aware of Paychex's 401k service, others went all out and tried to get their payroll clients on to the 401k platform as well.
Hope this helps.

Bill