Mo. Rev. Stat. § 103.098

There shall be a thirty-day enrollment period, at a time designated by the board, during which retirees and surviving dependents of retirees or employees of state agencies participating in the plan but not then covered by the medical care plan shall be able to enroll in the plan upon provision, at their * own expense, of evidence of good health satisfactory to the board. A preexisting condition will not be covered until a person has been a plan participant for twelve consecutive months.