How can I add custom metadata in the file/folders ' metadata?

[Steve M.]

You can't.

I would generally recommend keeping a mapping in your own database of file IDs to whatever custom metadata you want. It might help if you explain your scenario a bit more.

[Greg-DB]

The Dropbox API now offers the ability to add arbitrary key/value data to Dropbox files via "file properties":

https://www.dropbox.com/developers/documentation/http/documentation#file_properties

If you're using an official SDK, there will also be corresponding methods for these endpoints.