Can Plone display content from another site inside it?

Yes, there is a product that will do exactly this. It’s called windowZ. Just use the quick installer to set it up, and you can add Window objects through the Plone interface. Window objects don’t have their own content, but rather a URL that points to the content you want to display.

Additionally, the external content becomes searchable within Plone’s powerful search tool. However, if the content is constantly being updated (i.e. a Blog) this will not be automatically updated. Also, the external content must be of fixed width and height because WindowZ does not support dynamic data.