Bibledit can import USFM files.
In the menu choose File / Project / Import / USFM or other formatted text.
Select the directory that contains the existing USFM files.
In most cases it is enough to then click OK.
The books will be imported.
Also raw text can be imported. See Checks / USFM.
It can also import compressed archives containing importable files.
Apart from that backup and restore can be used.
Finally, books can be imported through the collaboration mechanism.
See also: Import, Import text dialog.
Some standard texts have been provided by the community. These are listed on the Bibledit wiki. See the section about the standard texts for import.