Adding a new file as if it had existed

Hi,

When I initialise a git repository, I use a subset of files in the
project and leave out irrelevant files for performance reasons. Then
when I need to make changes to a file not yet in the repository, the
file is treated as new, and if I reset the change or change branches
the file is gone.

Is there a good way of adding new files to git as if they had existed
from the initial commit (or even better, since a particular commit)?
This way I would only track the new changes I made to an existing
file.

Thanks,