Thank you for the additional information.
While I understand the basic issue that you are facing, I am not sure that reopening your pre-existing Chapter 13 is going to be an appropriate, or effective, mechanism to resolve your issue.
-You say that you have an agreement with the creditor to continue paying the loan, which you have reiterated currently, this can be construed as a "reaffirmation" agreement - meaning that the discharge would not be effective anyway.
-You are facing a potential foreclosure, this means that the creditor is likely to be claiming some significant amount of money - if you reopen your bankruptcy, the court is going to re-calculate the payments owed, and require that you make payments in proportion to the amount due to that creditor and their class. (You may be able to argue that your payments made during the bankruptcy should offset this).
-As far as the actual mechanism to do this, it really isn't all that complicated, you make an "ex parte" motion to the bankruptcy court to reopen the case and ask them to allow you to reopen the case to list an omitted creditor, explain to the court how the creditor came to be omitted, and what your projected changes to the schedules/payments will be.
-However, given the nature of the issues you are trying to deal with I would highly recommend that you speak to a local bankruptcy attorney to represent you (you aren't paying them to walk you through the process, you are paying them to help present the argument in the best possible way to have the court actually accept your position and grant the set aside to allow you to either do a cram down retroactively, or do some other mechanism of dealing with this junior lien - frankly, what you posted here, this is going to be a hard argument to make on a motion to reopen, and you need a lawyer to help you with this part).
-There may be other options to assist you, you mention there may be issues with the loan documents themselves (it is difficult to prevail on a claim solely based on a clerical error, but depending on the magnitude of what is at issue, you may have enough to attack the lien as a "cloud on title" (but be aware, that minor errors in title can be corrected by the creditor)). Again, local counsel is going to be your best source of information here, because you are going to need a formal legal opinion on this matter (interpretation on the deed, loan, and any interpreting documents).