You should be able to transfer a rollover IRA to your latest employer's 401k (and I think a regular, non-Roth, non-rollover IRA as well). One could probably enjoy unrestricted security choice of the traditional brokerage, then move funds back to the 401k before doing a backdoor Roth.

That said, I would talk to a financial advisor before moving large sums of money between account types. My 2c.