In the Visual Studio Add-in 10.1 project, open the Config.esriaddinx file and change the version to 10.0. Then recompile the project, select 'No' to ignore the 'update'. The newly created add-in file ('esriAddIn') can be used to install on ArcGIS 10.0 system.
But, again, if there's anything in the addin using new 10.1 functionality, it won't work on 10.0.