suppletory

suppletory \SUHP-li-tawr-ee, -tohr-ee\, adjective:

supplying a deficiency.

Every constituent is bound by suppletory rules in the charter, but each constituent is free at any time to alter by contract how a particular suppletory rule will apply to its positions and interests.
-- David Sciulli, Corporate Power in Civil Society, 2001

A book of accounts kept by one who has since become insane, and proved to be in his handwriting, is admissible in evidence, on being verified by the suppletory oath of his guardian.
-- Massachusetts Digest, 1881

Suppletory comes from the Latin word supplēre meaning "to make complete."