Thanks for the help Francisco.
I assume that the tank could not be imported from AutoCAD to STAAD.Pro, because it probably was not in the form of a mesh of plates with corner nodes. This is the way STAAD.Pro approximates a cylindrical structure like a tank.
I understand that you have successfully imported the support structure, and you have successfully modeled the tank.
1. Start with the model that contains the tank.
2. Select all of the tank plates, and then use Edit > Copy.
3. Move to the model with the support structure.
4. Use Edit > Paste and specify the appropriate shift in the Paste with Move dialog.
That should merge the two models together.
from one model to the