make .tgz package from installed package without rebuild?

[scole_mail <scole_mail%gmx.com@localhost>]

Is there a way to generate a package .tgz file for use with pkg_add
from an already installed package without rebuilding?
 
I would like to use this for example if I built and already installed
some time consuming package (e.g., firefox), and already did a "bmake
clean" and later decide I want to make a .tgz package file.

I guess something could be done with tar and pkg_info -L but wonder
if there is a proper command for this.

Thanks