A backdoor Roth IRA is not an account, it is a process. You make a non-deductible contribution to a traditional IRA, then convert it to a Roth IRA.
This process does not work if you have any pre-tax money in any traditional IRA, because all IRA accounts are grouped together for the tax and rollover rules.
If you currently have no pre-tax money in a traditional IRA, and you rollover your pre-tax 401k into a traditional IRA, you will not be able to do any more backdoor Roth IRA conversions unless you convert all your pre-tax IRA to a Roth IRA and pay the required income tax.
