Sure. At the bare minimum, you write "manual" `React.createElement()` calls. To simplify or shorten things, you can alias `React.createElement()` to something like `h()`.
There's also a variety of libs to help with that process. We have an existing ES5/Backbone app that's built with Require.js, and we don't have a "compile" step in there. We just added our first couple React+Redux-based features into that codebase, but can't use JSX due to that lack of compilation. I opted to bring in https://github.com/Jador/react-hyperscript-helpers as an alternative to JSX. Not perfect, but it's a suitable alternative given our limitations. There's several other "virtual DOM/hyperscript"-ish packages out there as well.
There's also a variety of libs to help with that process. We have an existing ES5/Backbone app that's built with Require.js, and we don't have a "compile" step in there. We just added our first couple React+Redux-based features into that codebase, but can't use JSX due to that lack of compilation. I opted to bring in https://github.com/Jador/react-hyperscript-helpers as an alternative to JSX. Not perfect, but it's a suitable alternative given our limitations. There's several other "virtual DOM/hyperscript"-ish packages out there as well.