The ftp command set is not the same as the unix command set though there are many commands with the same name. The ftp command set is in "man ftpd" for the target computer.
I personally have never seen a "chown" command in ftp but I have also never needed the command within a ftp environment. You usually ftp files to the account of the future owner.
Some versions of ftp allow more commands as parameters to the ftp "site" command but again I have never seen "chown". To my mind it would be a basic security hole.
I'm working on Drupal and I need to assign the ownership of some folders to Drupal. (Owner: _www)
After I copied them to the server the ownership changed.
Typically, for reasons of security, this is not something that an ftp server will allow you to do. If this is something that you will need to do on a regular basis the administrator of the ftp server can configure the server so that the correct ownership and permissions are assigned to the uploaded files.