Sorry, I missed the second half of the question,
"If so, then does it mean I have to transfer my rollover IRA back to a 401(k) to bypass the IRA Aggregation Rule?"
Normally, you can't take money out of a 401k unless you are retired or otherwise separated from service with the employer. So I am going to assume that you no longer work for this former employer.
If you work for a new employer that also has a 401k, they may allow you to rollover your IRA funds to their 401k. In that case, you should be able to rollover all your pre-tax IRA funds if you want to, not just the funds that came from the former employer.
I think it is less likely that your old employer will allow you to reopen your 401k if you closed it, but you could ask.
And yes, getting the money back into a 401k will avoid the aggregate rule affecting your backdoor Roth IRA conversion.
