Unable to add ProjLedgerPostingDefinitionEntity to a project in Visual Studio.

Hi,
I am working on D365FO(Update 13) and my requirement is to override a method mapDataSourcetoEntity of entity ProjLedgerPostingDefinitionEntity. According to my understanding, its possible through over-layering. So, created a new model by
selecting existing package ApplicationSuite during create model process and a new project. But, I am unable to add ProjLedgerPostingDefinitionEntity to this newly created project i.e option "Add to project" is greyed out for this entity.
Any suggestions are highly helpful.

Thanks,

Mounika