You can drag them into Photos but if it's a lot of files (thousands) Photos has a tendency to hang/crash. I'm the author of a free open source tool,
osxphotos, than has an import command to bulk import. For example: "osxphotos import /path/to/files --walk". It requires some level of comfort with the command line but gives you a lot of options for controlling the import, applying metadata, etc.