A bill that the Senate is expected to approve this month would give the 2.3 million federal workers covered by the current civil service retirement program until the end of 1987 to decide whether to join one of the two proposed new pension plans.

The bill, sponsored by Sens. Ted Stevens (R-Alaska) and William V. Roth (R-Del.), would establish two new retirement systems for federal workers hired since the end of 1983 who are now covered by both civil service and Social Security. Coverage under one of the new systems would be mandatory for those employes, and would begin in January 1987.

Persons already covered by the regular civil service retirement plan could stay in it until they retire, or would have until Dec. 31, 1987, to decide to go into one of the two new plans. If they elected to go into one of the new systems they would no longer contribute 7 percent of their salary to the civil service retirement fund, but would be required to contribute 7.05 percent to Social Security.

Workers who switched to the new plans would have their current benefits frozen, and could still retire at age 55 after 30 years. Their annuities would be based on the highest three-year average salary for time served under the current civil service system, and on their highest five-year salary for time served under one of the new options. Both the high-three and high-five computations would be based on the same highest average salary figure.

Both of the new plans are designed to pay employes Social Security benefits and scaled-down civil service pensions. They also offer workers the option of putting a portion of their salaries (matched partly by the government) into tax-deferred investment programs. Both options have the government making all contributions to the retirement fund and require employes to work until age 62 to get full benefits. One of the options would allow employes to retire at age 55 after 30 years (as under the current system) on full benefits if they contribute 1.3 percent of their salary toward the pension program. That contribution would drop slightly in future years.

If the Senate approves the bill it will go to the House. Democratic leaders in the House are working on their own supplemental retirement system or systems for federal workers who are covered by Social Security.