One other point that I haven't seen mentioned. Some 401(k) plans allow "after tax contributions" beyond the $17,500. In most cases, those same plans allow "in service rollovers" of the after tax contributions. You can roll these over to a traditional IRA, and then immediately convert it to a Roth IRA. This is a way to legally exceed the $5,500 annual cap on IRA contributions.
The sum of all 401(k) contributions (Roth 401(k), pre-tax 401(k), after-tax non-Roth 401(k), plus employer match to pre-tax 401(k)) may not exceed $51,000 for tax year 2013. Note that these are IRS maximums. Some 401(k) plans cap employee contributions at a percentage of their salary, but that's a 401(k) rule and not an IRS rule.
Here's an example, if your plan allows it:
$17,500 - "standard" employee contributions to pre-tax traditional 401(k)
$5,500 - employer match of pre-tax contributions to traditional 401(k)
$20,000 - "after tax" contributions to the 401(k). These contributions are in after-tax dollars (you could go up to $28,000 to hit the $51,000 max)
Next, you will do an "in service rollover" of just the $20,000 (plus any earnings) into a new traditional IRA that you set up just for this purpose. You many then convert that IRA into a Roth IRA.
See this for more:
http://www.irs.gov/Retirement-Plans/Plan-Participant,-Employee/401(k)-Resource-Guide---Plan-Participants---Limitation-on-Elective-Deferralshttps://forum.mrmoneymustache.com/investor-alley/401k-after-tax-to-roth-conversion/