Since 5.0.0 Guppy it is possible to install a plugin or a fork from the admin of the site.
See the online help
Warning: Guppy gives you the maximum size of the file to download, if your site is hosted by Free, you will have as maximum size 2Mb, it means that you can not use the install function for files <= 2Mb
If the archive is more than 2Mb, you need to install, unpack and put online by ftp.
See this tutorial for Lavachequireve guppyed.org