This may help answer your question, yawdert.
Do TSP Contributions Affect an IRA? | Finance - Zacks
basically, the benefits of doing what you propose may depend on whether your trad ira contributions are deductible or not. If they are non-deductible due to your income level, then rolling the non-deductible trad ira contributions might cause you to end up paying taxes on them twice, once when you put them in the non-deductible ira, and second time when you pull them out later from tsp. at least that would be my prognosis. perhaps best to consult a tax advisor on that one, I'm not one.
I rolled my non-deductible trad ira into roth-conversion account years ago when market was low (early 00s), had to pay taxes because I also had a separate older trad ira (deductible) account. to do that particular rollover, I had to calculate total (deductible+nondeductible) and pay taxes on proportion of total that I rolled that represented deductible part of the total trad ira, even tho I kept them in separate accounts for tracking up to that point. be careful, check for tax consequences now and later both. If you roll nondeductible trad ira into tsp, you will have to pay taxes now on the nondeductible contributions to trad ira, and then pay taxes again later when you pull them out of taxed tsp. I think.