I wrote an essay that I really like for a supplement on the CommonApp. I want to reuse it for a college that is not on the CommonApp. I would send it directly to them via their application portal. Can I do this? Will it seem like it’s not my own work?
Of course you can, as long as it is your own work and you wrote the essay. Perhaps I am misinterpreting the question, but I don’t understand why you are concerned about this. Most students recycle the essays they write for one college for another college’s prompts, with minimal editing. As long as it is your own work, you can send it to another college, even one that is not on the CommonApp!
Thank you so much!! Phew! Sorry if it seemed obvious, but I’m an international student, and I didn’t get a lot of help when applying. 
Yes, you can. Be sure to check the word count, and also make sure you are answering the prompt correctly for the college not on Common App.
Assuming it actually is your own work, I see no issues with this.
Just be sure it is addressing the essay prompt and putting forward what you want the admissions committee to know about you. Force-fitting an off-topic response to save writing an essay wouldn’t be a good idea.