5 CFR §1620.34

1. (a)An employee of a NAF instrumentality who moves from a NAF instrumentality to a Federal Government agency and who elects to be covered by a NAF retirement system is not eligible to participate in the TSP. Any TSP contributions relating to a period for which an employee elects retroactive NAF retirement coverage must be removed from the TSP as required by the regulations at 5 CFR part 1605.
2. (b)An employee of a NAF instrumentality who moves from a NAF instrumentality to a Federal Government agency and who elects to be covered by CSRS or FERS will become eligible to participate in the TSP as determined in accordance with 5 CFR part 1600.