The board of trustees of the Public Employees’ Retirement System is hereby authorized and empowered to include as a coverage group under this article those regular full-time civilian employees of the Mississippi National Guard whose entire salary is paid on certification out of allotted federal funds, provided funds are made available from the federal government or state appropriations to pay employers’ contributions on the salaries of such employees. In event the employers’ contributions on such salaries should fail to be paid in full to the public employees’ retirement system, the board of trustees of the Public Employees’ Retirement System shall be prevented from allowing service credit for any such period of delinquency and such retirants would be allowed only such service credits as had accrued up to the time of any such delinquency; but members who remain in the system with their service credits suspended during any such period of delinquency may obtain full service credit upon the payment of all employers’ contributions due the retirement system for the entire coverage group.