Can anyone explain how to install an application that does not exist in the freedom box?
Does anyone know how to install and run an open Linux diaspora on the Freedom box?
Hello @boxkey
Thanks to your interest and welcome here 
About how to add a app, you may want to read HACKING.md and FreedomBox Developer Documentation.
About diaspora debian package availability, it seems to be too early for now. I don’t know myself about hardware requirement to run on some SBCs.