Our company is working on a certain version of an application with two divisions. The main division that originally made the application uses Perforce as version control and we use Git.

We need to develop some separate features and got a zipped version of their code. We created a new Repo and added their code to safely work together on it in our division with multiple people. The other division will send updated versions again through a .zip file.

I already tried making a new branch, replace our local files with theirs and push those changes. But that seems to overwrite all of our existing features we added.

How can we merge their new code into ours, without deleting all the changes/features we made?