You can, but it will not stay set to the value you give it.
The FQDN attribute is what is known as an automatic attribute in Chef. It is set by Ohai each and every time Ohai runs.
Therefore, while you can change the value of the FQDN attribute, the next time Ohai runs, it will set it back to be the value returned by hostname --fqdn
.
Therefore, as noted above, if you want to change the FQDN attribute permanently, you will need to change the hostname.