You will run into a problem with a backdoor roth iRA because you have a rollover IRA.
Let's say you have $100,000 in a traditional rollover IRA.
And you want to put $6000 into a "backdoor" roth.
You would fund it by putting it into a traditional IRA, but declare it be an "after-tax" contribution.
Then ideally, you'd immediately convert the traditional IRA after-tax contribution into a ROTH.
And this is how it would work if you didn't have the traditional rollover.
But....The IRS requires a proportional conversion of after-tax and pre-tax dollars.
So, in the case above:
IRS would require you to move a proportional amount from each account.
I think the math would go something like:
6000/(100000+6000) = after-tax percentage allowed to be rolled over.
So, when you went to do the rollover, you'd be able to move 5.6% of your after-tax contribution ($340).
And the other 94% would come from pre-tax dollars ($6000-$340) = $5660.
This would unfortunately force you to declare that $5660 as income and pay wage taxes on it your highest marginal rate.
The only way I could figure out to get around this was to convert my rollover IRA back into a 401k via my current employer.