yes, you can.
There is a process to manually add files, then have nextcloud to scan those files to add them to the database:
copy/past teh files you want to add to the final location/folder/users you want to use. Can be made by whatever way you want, i parsonnaly used ftp.
using ssh, you have occ command to scan/add them to the database.
as said in doc : https://docs.nextcloud.com/server/9.0/admin_manual/configuration_server/occ_command.html#file-operations-label
occ has three commands for managing files in Nextcloud:
files:cleanup cleanup filecache
files:scan rescan filesystem
files:transfer-ownership All files and folders are moved to another
user - shares are moved as well.