The offer to non-members, who include some who refused to join the pounds 10.5bn scheme when they arrived at the Post Office, runs to the end of June.
Only a handful of public sector employers have made arrangements to readmit pension scheme members who left as a result of the personal pensions mis- selling scandal, which came to light more than two years ago.
Members of schemes such as the mineworkers, the teachers, the nurses and the Post Office were tempted by hard selling to drop the often substantial benefits of their corporate schemes and take up personal pensions that offered less security and lower pensions.
The Securities and Investments Board stepped in when the scale of the problem came to light, and pension providers are expected to pay substantial damages to customers who were sold the wrong policies. But so far no cases have reached court and no compensation has been paid.
The Post Office said "The amnesty gives our people a second chance if they feel they made a mistake, perhaps after being targeted by personal pension sales teams. Many responded immediately to our offer and more are following as they calculate the benefits."
Under the rules of the scheme, many of the 11,000 employees are prevented from rejoining by age or time limits, but the Post Office said the present circumstances were unusual.
A survey by the Post Office found that four out of five of its employees who opted out of the scheme did so to take out personal pensions, but the majority regretted it and now wanted to change their minds and get back in.
Two-thirds of those who had never joined the company scheme also wanted a second chance to become members. Although the 11,000 will be able to regain the benefits of the Post Office's generous state sector scheme, they will not be able to replace the benefits lost during their period outside the scheme, except by suing the personal pension providers for compensation.
Among the other state employees hit by the scandal, the Home Office has agreed to amend legislation that prevents the 400 police who left their pension scheme rejoining.