I don't know if it is possible to import the calculated items all at once.
If you have a huge number of calculated items, you might want to save the original report as a copy and work from that when you want to make a new report.
You can easily copy calculated items by entering the Text mode in the Edit Calculated Item window. Then you can copy the expression for the calculated item and paste it into your other report. If you copy the expressions for all your items into a text file, that would minimize the back-and-forth between reports.