Small Employer Defined Benefit Expansion Act - Amends the Employee Retirement Income Security Act of 1974 (ERISA) and the Internal Revenue Code to set forth special rules for treating the combination in a single pension plan of: (1) a defined benefit plan (a traditional benefit plan, or else a cash-balance plan for which additional special rules are required); and (2) a defined contribution plan (an individual account plan which includes a cash or deferred arrangement).