Yes, as long as you meet the income requirements for contributing to a Roth IRA. It’s a common misconception that people cannot fund an IRA if they have fully funded an employer plan like the TSP. Nothing could be further from the truth.
You still have through April 15, 2019 to contribute to an IRA for 2018, and the 2018 IRA contribution limit is $5,500, with an extra $1,000 you are 50 or over (this includes the year in which you turn 50).
Unlike the TSP, there are income limits that might restrict your ability to contribute to a Roth IRA. If your filing status is single, you may fully fund a Roth IRA if your adjusted gross income is below $120,000. If it’s joint, you can fully contribute if your AGI is up to $193,000. You cannot contribute at all if your AGI is above $137,000 (single) or $203,000 (joint).
You can find more information about IRA contributions in IRS Publication 590A.