There are similar things that a 401k can do (it's called a mega backdoor roth), but no off the shelf solo 401k providers allow the types of things required (after tax -- not roth -- contributions and in service rollovers or withdrawals), so you would need to have a self managed solo 401k to do it. Assuming your solo 401k is provided by fidelity, vanguard, td ameritrade, etrade, etc then the practical difference is that you just won't be able to do it.
If you want money in a roth 401k, then just put it straight in as there are no income requirements. Not all providers allow roth contributions. The $18k limit is shared between roth and traditional though, and profit sharing can only go into traditional.
Anyone can do a regular backdoor roth utilizing IRAs. This is only useful because there are income limits for roth IRA contributions, and for deductible IRA contributions, but not for non-deductible IRA contributions. Non-deductible contributions aren't very useful except in as much as they can be rolled over into a roth. Hence the contributing and rolling over machinations instead of just contributing straight to a roth IRA as lower income folks can.