PHP part in Python based website
Quercus on App Engine runs on the Java runtime, while Python runs on, er, the Python runtime. While you can run one app with multiple runtimes, they have to be different major versions, which means that communication between them will be awkward - you'd have to make URLFetches between them.
I think you'll find it's far simpler to learn enough Python to make the changes. And trust me, you won't regret learning Python. ;)
I need to write a small addition to a Python+Django based website, hosted on Google Apps. Trouble is that I don't have any experience with Python.
The addition is pretty straight forward, it requires little database access
So  thought of 2 possible solution's:

Quickly dive into Python and get it done with Python.
Run PHP on Quercus and write this particular addition in PHP, then somehow combine it into the website as a dingle PHP page.

Edit: There is another option I thought of:

Writing it in PHP and hosting it on an external server, embedding it in an iframe. The addition I am writing has no need to be SEO friendly, so an iframe will have no negative effect on the website.

What do you suggest?