This situation is complicated and we don't have a great way of handling this now in the software. The best way to handle right now is to tread the non-load bearing riser as a general attachment. This will allow you to add weight to the structure where you wish for it to be added. The attachments only add weight, they do not add stiffness. You can specify that the wind area is zero since it's inside the outer pipe.
Where it gets complicated is in how you are going to handle the internal riser weight. If the weight is all supported at the base, then you would ideally apply the weight as a point load at the base of the outer riser; however, this will not adequately represent the mass of the inner riser along the entire height in order to get an accurate mass distribution for the dynamic analysis (Natural Frequency). If this is a concern, then the best option that we can offer at this time is to make two models.
1) One will uniformly distribute the weight of the internal riser along the entire length, and this model would primarily be used to verify the frequency of the structure.
2) The 2nd model will be with the load applied at the elevation where it is supported. This is primarily used for the analysis for dead load and lateral loads (wind/seismic).