I don't really understand your question, I don't know anything about an rss.xml file there is no such thing in mojoPortal
There are only 3 places in mojoPortal where RSS is exposed as an outgoing feed.
The RSS Module can consume feeds from external sites or from internal features that expose RSS.
The blog module exposes RSS and provides its link, if you mouse over it you can copy the url and consume that in the RSS Feed Module.
The Forums module can also expose content as RSS if you enable it in the Module settings by clicking the gear next to the title of the Forums module. It will then expose RSS links that can be consumed externally or internally and you can get the url by mousing over the link or right clicking it.
The RSS Feed Module can consume 1 or more feeds and it can expose an outgoing feed that aggregates the ones that it consumes.
Thats all there is at the moment there is no general feature by which a page can expose RSS. I do plan to enable RSS outgoing feeds on more features in the future.
Hope it helps,
Joe