Texas Government Code - GOV'T § 842.107. Optional Membership

(a) A subdivision may authorize to be a member of the retirement system a person who is accruing benefits in another statewide retirement system for service in an elected or appointed judicial or district office or as an employee of the state or a governmental unit of the state during the same period the person is receiving supplemental compensation from the subdivision. A person described by this subsection who is first included for optional membership after December 31, 2005, may not contribute to the employees saving fund or receive any service credit for any supplemental compensation received before the date the subdivision makes the person eligible to become a member.

(b) The board by rule may establish reasonable restrictions and limitations on the granting of membership and service credit under this section.