https://www.tsp.gov/PlanParticipatio...igibility.html
Eligibility Requirements

The TSP requires that:

  • You have an existing TSP account (I believe you have at least $300 in tsp account for it to consider active). You cannot open a TSP account by transferring money into it.
  • The money that you intend to move is considered an "eligible rollover distribution" for Federal income tax purposes. You can verify this by checking with the administrator of the plan or IRA from which you are moving the money. You can also consult a tax advisor.

The TSP will accept into the traditional balance of your TSP account:

  • both transfers and rollovers of tax-deferred money from traditional IRAs, SIMPLE IRAs, and eligible employer plans.

The TSP will accept into the Roth balance of your TSP account:

  • only transfers (i.e., direct rollovers) of qualified and non-qualified Roth distributions from Roth 401(k)s, Roth 403(b)s, and Roth 457(b)s. If you don't already have a Roth balance in your existing TSP account, the transfer will create one.

The TSP will not accept Roth rollovers that have already been paid to you and it will not accept transfers or rollovers from Roth IRAs.