You can use the external storage storage support app. Enable that and perhaps install inotify app. That way you can add other folders to use with Nextcloud.
Is this only for a single user or do you want this to allow multiple users access to these files?
In the latter situation I would recommend to share the files/dir from one user with the other ones instead of having all users adding the same external storage, since this would cause some infernal overhead that those files create multiple file cache entries for each user and is scanned multiple times for each user as well.
IMO in most situations it is preferred to keep all files inside there data dir when anyway on the same machine, and use the sharing feature.