I have applied for service credit and am waiting to find out the amount due. Can TSP funds be used to purchase federal service credit?
Answer: No- you can't directly take TSP funds while working and use them to fund your military service credit . You COULD take out a general TSP loan, but there really is no benefit to doing that, UNLESS you are trying to retire immediately and don't have any other choice. In that case, yes, you could take out the loan, but then you'll be hit with the taxable income if you retire before paying off the loan.
Under the rules for FERS employees, if you make your military service deposit within three years of starting federal employment, all you have to pay is the 1.3% of military pay, and no interest is due. If you wait until later, you'll have to pay the 1.3%, plus interest each year.
Most people who do the service credit do so by making a per-paycheck deduction. Just make sure you put in enough to pay down the principle. It has been known that some folks barely contribute enough and stretch it out for years, ending up paying more in interest than they would have in priniciple.
Here is a link to the OPM rules for Military FERS deposit credit:
Creditable Service
Make that deposit within the first three years of federal service, and no interest accrues. Good luck.