Retroactively Creating a Child Theme?

I inherited a site that has lots of customization and no child theme. At this point is it possible to just take the current theme, call it a child theme, and install the new updated theme as normal?

This method seems as if it should work just like installing the original theme, making a child, customizing the child, and then updating the original theme as time goes on.

Any thoughts on this? The theme is Decode. Thank you.