No Child Theme and Functions.php

Hi all -

New to GP here, so I apologize in advance for the dumb question. I'm not new to WP but I'm by no means a developer, etc. I realize the importance of "Child Themes" in customizing things but I've run into a small issue that I need some help/advice on:

• I like the way the base GP theme works. I'm building a quick, simple, private HOA blog for a small neighborhood. There's no child theme here...and it didn't hit me that I might have backed myself into a corner until...
• I needed to add some code so that my manual excerpts generated a "Read More" button. When I went to paste the code (from this forum), I realized... "crap. this isn't a child theme...I might have screwed myself here..."

What I need some help with is - can I keep what I have done so far and make a child theme? I'd really like to have a button after my manual excerpts but I realize, if I stay where I'm at, I'm going to screw myself every time GP updates.

Thanks for any help.

-Dumbass