Welcome to our community, Jay
Yes, it's possible to combine projects. Keep in mind that
projects DO have a certain size from the "number of total slides"
perspective that is considered best practice. That magick number is
typically between 50 - 65 (give or take and subject to all manner
of interpretation). So if your new combined project will be way
past that, you may wish to reconsider the approach.
To combine, click File > Import > Slides/Objects and
point at the other project.
Also, it will keep things simpler if you have both projects
sized identically.
Cheers... Rick