Member employers which provide retirement benefits under a local or special Act may contract with the board of trustees for transfer of credits to this retirement system and continuation of accrued benefits to their employees under terms which may be provided by amendment to such local or special Act and which are not inconsistent with this chapter or rules or regulations adopted pursuant to this chapter. Otherwise, this chapter is not intended to affect existing retirement or pension systems established by local or special Act or to preclude the establishment of individual employer retirement systems by future local or special Acts.