Can a Roth IRA be rolled/transferred into a Roth 401(k) plan?

No. I think there was a recent thread on this; don't remember if it had a cite or not - it's probably a case where there simply is no cite permitting it.

No. Roth 401(k) can go to Roth IRA (or another Roth 401(k)), but not the other way around.

…because the rollover rules prohibit the rollover of after-tax assets from IRAs to qualified plans. IRC Sec. 402©(4)

That cite is for Eligible Rollover Distributions - going out of a plan - so I don't see how it applies.

The code does permit plan to plan and/or plan to IRA rollovers of Roth accounts (402A©(3)).

Now I'm convinced that you can't do a Roth IRA to Roth 401(k) because there is no cite saying you CAN, not because there is a cite saying you CAN'T.

FWIW.

No cite says you can = you can't. The default for any distribution is immediate taxation. If there is an exception, you can use that narrow exception. If there is not an exception, it does not mean there is a hole that no one ever thought about, it means that Congress thought about it and wanted the distribution to be taxable and made it taxable by not creating an explicit exception to the general tax inclusion rule.

Huh? Oh, "There is no cite saying you can, therefore you can't." OK, we agree.

I'm not so sure I would agree that Congress thought it through that carefully...