Trustmark Insurance Co., Lake Forest, Ill., plans to add the Oakmark Select One Fund as an option in its $68 million 401(k) plan, and replace the Morgan Stanley Institutional International Magnum Fund with the Morgan Stanley Institutional International Equity Fund, said Frank Cavanaugh, associate director of human resources. The board is expected to approve the changes by July 1, increasing the plans total number of investment options to nine from eight. The Oakmark fund will give participants a midcap option, and the Equity Fund has consistently higher returns than the Magnum Fund, he said. Fidelity is record keeper, trustee and service provider for the plan. Administration is handled internally.