Mo. Rev. Stat. § 473.423

A personal representative may establish a claim against the estate by proceeding against his corepresentative in the manner prescribed for other persons; but if there is no corepresentative, he shall file his claim and other papers, and, unless the persons whose interests would be adversely affected consent thereto in writing, the court shall appoint some suitable person as administrator ad litem to appear and manage the defense. The fee allowed to the administrator ad litem shall be charged against the claimant as costs unless the claimant can show that the claim arose prior to decedent's death or, that by advancing funds on behalf of the estate, the estate was benefitted thereby, in which event the fee allowed the administrator ad litem shall be charged as costs against the estate and paid as an expense of administration.