How do I properly add a custom source for RSS feed import?
The RSS/Atom parsers only work with predefined sources. The custom sources in this case are only useful when using them in combination with Feeds Tamper (or alike).

If you want more control about selecting data from a XML file, try the XML parser from Feeds extensible parsers instead. With that parser, you can select data from a XML file using XPath queries. So you are free to select about anything from a XML file.
It would be good idea to learn more about XPath before using the XML parser. I think that the XPath tutorial from W3Schools is a good resource for this.