You can import a folder from On My iPad or iCloud Drive, or a connected USB drive, but Apple’s Files app does not allow folders to be imported from third party cloud storage providers, such as Dropbox. This is covered in this important video about importing evidence: https://support.litsoftware.com/hc/en-us/articles/360058503294-VIDEO-Importing-Evidence
You can get around this limitation by compressing the folder into a single ZIP file first, and then importing the zipped folder as a single document.
Do this by opening the Files app on your iPad, tap on the the third party cloud storage provider and browse to the folder you want to import, then tap-and-hold on the folder and choose Compress from the popover. This will create a single ZIP file of the folder which you can then import using the Import Documents option. When this file is imported into TrialPad or DocReviewPad, it will uncompress the files and reconstruct your file and folder structure within the app.