Wash. Rev. Code § 41.40.056
Except as qualified by RCW 41.40.023, for employers that were admitted into the retirement system before July 23, 1995, membership service may be established for the employer's former employees who are active members of the system if the member or member's former employer pays an amount equal to the employer and member contributions which would have been paid to the retirement system on account of such service to the retirement system. Payment shall be made prior to the retirement of such member.
Payments submitted by the member under this section shall be placed in the member's individual account in the members' savings fund and be treated as any other contribution made by the member, with the exception that the contributions submitted by the member in payment of the employer's obligation, together with the interest the director may apply to the employer's contribution, shall be excluded from the calculation of the member's annuity in the event the member selects a benefit with an annuity option.
RCW 41.40.056