That's what the https://www.investopedia.com/terms/b/backdoor-roth-ira.asp is for, assuming you don't have any existing traditional IRA balances

I have considerable Roth assets because my employer's 401k allows for the Mega-backdoor, which means I can put $30k+ per year of after-tax income into 401k (beyond the normal pre-tax contributions) perform a Roth-in-plan-conversion on the after-tax assets, and then roll it out into a Roth IRA.

You can convert your 401k to an IRA when you leave an employer. Some employers also offer in service rollovers (I think these mostly have minimum age restrictions on them though)