You can nest two git repo's without using submodules. Suppose ChildRepo is a subdirectory of ParentRepo, and both are git repositories.
+ ParentRepo
- ChildRepo
If you add a file to ChildRepo, ParentRepo will ignore it. When you commit it it will be added to ChildRepo. It is not possible to add files within ChildRepo to ParentRepo.
More info: Nested GIT repo gotchas!