Create outlines/TOC for existing PDF in Python

I'm using pyPdf to merge several PDF files into one. This works great, but I would also need to add a table of contents/outlines/bookmarks to the PDF file that is generated.

pyPdf seems to have only read support for outlines. Reportlab would allow me to create them, but the opensource version does not support loading PDF files, so that doesn't work to add outlines to an existing file.

Is there any way I can add outlines to an existing PDF using Python, or any library that would allow that?