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.
Then you will have to got into imscp and do an update, so the plugins will appear.
Settings -> Plugin Management -> Update Plugin list