Hi there,
As what @rade_tomovic said, you can add criteria to your Search Sets in the Find Items window. You can use the OR condition so that the original selection wont be affected.
I tried to simulate your workflow and here's what I did:
First I exported an external data source (in this case a CSV file) then built a Task Hierarchy so that the data will be exported in the Tasks tab. In the Tasks tab, I selected all the tasks to be exported to Selection Sets.
Then, I created Search Sets for the Tasks, utilizing the Find Items window to create them. When clicking on the Search Sets in the Sets window, the elements get selected in the model space. I then selected the Selection Set based from a certain Task and did Update. Since Search Sets and Selection Sets cannot be linked together, this is the only thing I can do.
Now, if there are things not included in your Search Set that needs to be included, I go back to my Search Set by clicking it and go to my Find Items window to add conditions. I use Or conditions to ease up the searching of elements. I then Update my Search Set. While the elements are selected I go back to the corresponding Selection Set and Update it as well.
I hope this helps.