Legislation that would set up a new mandatory retirement system for federal workers hired in or since January 1984 is expected to be approved tomorrow by the House Post Office-Civil Service Committee.

The bill, sponsored by Reps. William D. Ford (D-Mich.) and Mary Rose Oakar (D-Ohio), would create a new pension program for the 300,000 employes hired since January of last year. Their future retirement benefits would be based on Social Security, a modified civil service retirement package and earnings from an optional tax-deferred investment program.

Employes could retire at age 55 after 30 years, under the House plan. Workers hired before 1984 could not come into the new system, but could take advantage of a portion of the thrift it would create.

Last week the Senate voted 96 to 1 in favor of two new retirement options for workers. Under the Senate plans, sponsored by Sens. William V. Roth Jr. (R-Del.) and Ted Stevens (R-Alaska), feds hired after 1983 could choose between one of two proposed plans that base benefits on Social Security, civil service and tax-deferred savings.

Workers hired before 1984 would have the option of remaining under the current civil service retirement program or of joining one of the two plans that would be mandatory for new hires.

One of the options would require employes to work until age 62 for full benefits. The other would allow them to contribute toward their retirement and retire at age 55 after 30 years.

Once the House committee approves the Ford-Oakar bill it can be sent either to the floor for a vote, or directly to conference with the Senate.

Congress has until the end of the year to establish a new program for post-1983 hires. If a new plan is not approved, next year those workers will have to contribute more than 14 percent of their salary toward Social Security and the civil service retirement program.