Just add more script steps after the import records step.
The problem with that: each import requires the user to choose a unique file path. So the script sort of "exits" once it gets to Import Records, and does not resume after the user selects the correct file path.
That should not be the case, the script should resume once the records are imported. Did you run an actual script or just select Import Records from the file menu.
Just to make sure, I created a small test file with this script:
Import Records 
Show Custom Dialog ["Hello"]
I ran the script, used the dialogs to select a text file, map fields and imported the data. Once I click Ok to close the import summary dialog, the custom dialog message opened as expected.