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.