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As explained in the plugins, in the recent versions of imscp there is a plugin system that allows extending the functions of the control panel. Here are the instructions on installing the plugins in the server.
The standard way to enable a plugin for the system is to upload the package containing it to the control panel via
Settings -> Plugin Management -> Upload
The package needs to be either in a zip., a tar.gz or a tar.bz2 file, there are several ways to get one of this plugins as a package.
There is another method that involves having a ssh connection to the server and a git client, for example to have all the plugins (the stable ones) you could also do.
mv /var/www/imscp/gui/plugins /tmp cd /var/www/imscp/gui git clone https://github.com/i-MSCP/plugins.git plugins/
Then you will have to got into imscp and do an update, so the plugins will appear.
Settings -> Plugin Management -> Update Plugin list