Job cutbacks in nondefense agencies and reforms of the federal retirement system will be the main civil service goals of the second Reagan administration, top officials say.

Although the federal grapevine is buzzing with rumors of a post-election freeze on hiring and promotion, insiders say it is more likely that the administration will continue its job-cutting efforts via attrition. Nevertheless, many federal agencies have stepped up efforts to fill vacancies in anticipation of a possible freeze.

"We have eliminated 75,000 nondefense federal jobs since 1981," an Office of Personnel Management official said yesterday, "and it would be counterproductive to slap a freeze on hiring now." (Some of that reduction was offset by increased employment in defense agencies.)

The OPM official said he anticipates that agencies will be told to make cuts during the next four years through normal attrition; that is, filling only some of the jobs that become vacant. Each year, federal agencies hire about 200,000 new workers, 40 percent of whom are replacement civilian workers for the Army, Navy and Air Force.

"If you want to know what we would like to do with federal retirement," he said, "read the Grace Commission report recommendations." That report suggests that federal workers -- who can now retire at age 55 with 30 years' service with an annuity equal to about 56 percent of salary -- be required to work longer to draw unreduced benefits.

Here is a brief rundown of likely items on the agenda: Agency budgets that have been tentatively approved may be pulled back by the Office of Management and Budget for new language and/or ceilings that would require agencies to make additional job cuts during the next year via attrition. A new legislative push will be on to require federal and postal workers to contribute more money toward their retirement. At present, most employes put 7 percent of salary into the civil service retirement fund. The administration would like to increase that contribution, over a two-year period, to 9 percent. An attempt will be made to persuade Congress to design a new federal retirement system -- for employes hired since last Jan. 1 -- that incorporates federal retirement and Social Security benefits.

If the administration has its way, that new system will require employes to work until age 62 before they can retire on reduced benefits, or until age 65 (the same as Social Security) for full benefits.

Democratic sources on Capitol Hill paint a much grimmer view of a second Reagan term. They predict that there will be massive cuts in nondefense federal employment and one staffer said, "a round of RIFs reductions in force that will make the last four years seem like a picnic." During the first three years of the Reagan administration, nearly 3,000 federal workers here (out of about 347,000) were fired.