Waterspeed can import many GPX types of files, to manually add activities from other sources or apps. Use it from the Diary page, from the 3 dots menu at the top right corner.

Import multiple files:
you can import multiple GPX files. Open the iCloud Drive folder from your Mac/PC or mobile device (or from Apple Files app from your iPhone). In the iCloud folder, Waterspeed, open the Waterspeed-sync folder and save files there.
the app will import as soon as you open the app. It might take time to complete.
This feature is available from the app version 6.23 (be sure to have updated).

Note: it is available only if you subscribed to Waterspeed Pro (see the related article for details).