I could not found any way to export/import all the user defined reports at a time. You can save the report one by one in host A and open it in host B. Here is the steps:
1. Right click on one of the report. Click on 'Save As'.
2. Save the file as XML and move the file to host B.
3. Open SQL Developer in host B. Right click on 'User Defined Reports'. Click on 'Open Report'.
4. Select the reports XML file and open it.
you can only export the Reports onle level below the folder "User Defined Reports". I usually create one folder per project/application to group my reports and therefore I can save/share them easily.
To save the complete reports, search for UserReports.xml in your working directory. When you replace the same file on host B you will have the identical reports. Of course you will lose existing reports there.