If you are modeling a supply pipe to a reservoir and you wish to consider the effects of the fluid in the reservoir on the calculated system, what you can do is set the fluid in the reservoir to water or air (or whatever your fluid is) and calculate on that basis.
We have applied this approach in the example below:
The top model has water in the receiving tank/reservoir and the second model has air. All other parameters are the same.
We can see in the results that the system inlet pressure takes account of the fluid in the receiving tank (takes account the density and head of the fluid in tanks 2 and 4).