Can the Conduit element be used to model a closed pipes that can operate under pressure?
The “conduit” element in PondPack essentially models open channels. More specifically, it models hydrograph translation and attenuation using Muskingum or Modified Puls hydrologic routing. If the water surface reaches the top of the conduit's defined cross section, an overflow condition is assumed to occur. You'll see a user notification and the hydrograph will be truncated.
For interconnected pond routing, it is also important to note that the water surface in the conduit does not affect upstream elements. In other words if two ponds are connected through a conduit, the upstream pond will not "see" the tailwater from a downstream pond through the conduit. The two ponds will need to be connected by a pond outlet structure. So, if you have closed pipe that operates under pressure, you would model that as a culvert in your pond outlet structure (pond 1 > pond outlet node > pond outlet link with culvert > pond 2.
If your situation is more complex, such as the pipes being long enough that you need to account for the travel time/attenuation as well as tailwater effects, you might want to consider our more advanced stormwater modeling package, CivilStorm. CivilStorm uses a fully dynamic numerical solver which can handle more complex situations automatically.