Looks like you're on the right track. In order to mega-backdoor, your 401k needs to allow after-tax contributions and in-service non-hardship withdraws.
If they do, your limit for 2018 is: $55,000 - $[Your Pre-Tax and/or Roth Contributions up to $18,500/24,500] - $[Company Match]
They will likely require you to distribute the earnings on the after-tax contributions as well. If your earnings are significant, ask if those can be distributed separately to a tIRA or rolled back into your 401k. If the earnings are relatively small I might just convert them to roth and pay tax on the conversion.