If you mean that you want to Reuse the Project Content then it might be Possible to Perforce Shared Stream although I have not tried that thing yet
But if you want to Add Content in All Projects then
then you can create use Custom Templates for new, and for existing Projects you could just Copy Paste that Content in your Project Content folder Directly from Vault Cache.
If you are using Plugin then you could set it Enabled by Default in .uplugin file in your Engine Folder/Plugin/Marketplace/Plugin you want to be enabled by Default.
You can use Content Only Custom Plugin, by moving required Content to Plugin Content Folder and using the Plugin method I mentioned to set it Enabled by Default so that you won’t have to Press Add to Project every time in New Project, Downside is you have to manually Update them but Plus Point is you have to Update once and it will reflect in All Projects.