New federal employes and postal workers would have two pension options to chose from, while those hired before 1984 could pick from three retirement plans under a proposal introduced yesterday by Sens. Ted Stevens (R-Alaska) and William V. Roth Jr. (R-Del.).

Congress has until the end of the year to come up with a supplemental retirement program designed to cover the 300,000 government employes hired since Dec. 31, 1983. Those employes are now covered by the regular civil service system and Social Security.

Any new federal retirement system will be mandatory only for the recent hires covered by Social Security. The more than 2 million feds who are already covered by the civil service retirement system could join the new plan, or stay in the current system.

The primary attraction of the current system is that it offers workers benefits fully indexed to inflation, ranging from about 53 percent of salary at age 55 (after 30 years) to 80 percent of salary after 41 years of service. Employes contribute 7 percent of their salaries toward the retirement program, and pay the 1.8 percent Medicare tax.

In July Roth and Stevens proposed a three-tier retirement plan for new workers. It is based on Social Security benefits, reduced civil service benefits and tax-deferred investments that would allow workers to put up to 10 percent of salary (with the government putting an additional 5 percent) into the program.

Federal and postal unions objected to the original plan because employes would have to work until age 62 to get unreduced benefits, and because they said the tax deferral plan would be of little benefit to low-income workers. They also object to giving future retirees raises that would be pegged 2 percent below the actual annual rise in living costs.

Yesterday's action would give new employes the chance to join the original Roth-Stevens plan, or have the option of getting into a new program that would allow them to retire on unreduced benefits at age 55, with 30 years service -- provided they contribute 1.3 percent of salary toward the retirement system.

That second option would provide bigger civil service benefits but reduce employe participation in the tax deferral plan to 6 percent of salary, and the government contribution to 3 percent. Employes also would have to contribute 1.3 percent of salary toward civil service retirement if they joined the second option, but would contribute nothing if they elected the first option.

The first option provides lower guaranteed civil service benefits, but the chance to make more tax-deferred investments. The second option proposed yesterday provides a bigger guaranteed civil service benefit but less investment chances and tax savings.

The Roth-Stevens dual option bill may clear the Senate shortly. But the Democratic-controlled House is expected to come up with a different supplemental retirement plan, along lines suggested by unions.

Such a bill would allow employes to retire at age 55 after 30 years, continue full cost-of-living raises to new retirees and keep the current formula, which bases pensions on the employes' highest three-year salary average, rather than the high-five formula contained in both Senate options.

If Congress fails to approve a supplemental retirement system before the end of the year, newly hired feds in 1986 would have to put 7 percent of their salaries into the civil service retirement fund as well as make the 7.05 percent Social Security contribution.