Yes, it is possible to contribute to both a 401k plan including a self-employed solo 401k and a nonqualified deferred compensation plan. Often times, a a nonqualified deferred compensation plan is used to help supplement a high earners retirement planning.
1) Having a Nonqualified Deferred compensation Plan (NQDC) will not prevent one from setting up a Solo 401k (if one is otherwise eligible to do so).
2) While we don’t offer non-qualified plans, to our knowledge contributions to such a plan won’t reduce one’s ability to make contributions to a Solo 401k since they are not tax deductible.
Items to Consider
Since contributions to a solo 401k are tax deductible and Roth solo 401k funds also grow tax free, one may consider first contributing to a solo 401k as contributions to a Nonqualified Deferred compensation Plan (NQDC) are neither tax deductible nor are distributions tax free once distributions commence.