Adding money to TSP after retirement

You are not allowed to make contributions to the TSP other than by payroll deduction, so, if your intent is to make regular payments to the TSP after separation, the answer is no.
Of course, nothing is that simple. After separation, even if you are withdrawing money from the TSP, you are able to make rollovers into the TSP from qualified accounts. If you qualify to make contributions to a traditional deductible IRA, you can rollover said IRA into the TSP.
In order to make deductible contributions to a traditional IRA, you must not have reached the year in which you turn 70 1/2, have at least enough earned income to cover the IRA contribution, and have a Modified Adjusted Gross Income below certain levels (see IRS Publication 590-A).