Maybe we are talking about two different things.
The OP already has a traditional IRA that was created from a rollover from a 401K at a previous employer. That IRA has a balance of $15,000 and a basis of $0.
If he creates another separate IRA (at Vanguard, Etrade, wherever) and funds it with $5500 of post tax money, it will have a basis of $5500.
If he chooses to convert that $5500 IRA to a Roth, he will owe tax because of the $15000 IRA that has zero basis. This is not speculation by me, this is hardcore IRS fact.
I was suggesting maybe he choose to take the pain now and convert everything to a Roth while the $15,000 IRA is relatively low balance if and only if he plans to do backdoor Roths for quite a few years.
The caveat is that they could stop the ability to do a backdoor Roth in the future, although I have my doubts that they will because it gives politicians money now (from the tax on some conversions).