No idea, how you would get them out. But then it depends on the
content type and the meta data you want to preserve.
You probably need to implement some kind of importer. e.g. for
documents, you would need to put the metadata + files somewhere and
then implement a service that reads metadata and files and uses
DLFileEntryLocalService (and maybe DLFolderLocalService) to create
files and folders in the system. If you need e.g. tags/categories, you
can add them too in that case.
https://docs.liferay.com/portal/7.0/javadocs/portal-kernel/com/liferay/document/library/kernel/service/DLFileEntryLocalService.html
If you need no metadata, you might even be able to import them using
webdav. Then it would be a simple copy & paste operation.
But it really depends on the details. It is really impossible to give
you an answer in a forum posting to such big question.
