I'm not sure if it is possible, but even if it is, the integration will be very basic and a lot of OOTB taskflows that work with WebCenter Content won't work for you.
Maybe, you could consolidate you WebCenter Portal and WebCenter Sites licenses into WebCenter Suite Plus, which contains also Content. Note that since .8 Sites may store its content in WebCenter Content, so it might be a clean solution with no or minimal costs.
Well , as suggested by jiri machotka, you can go for version 18.104.22.168.0 version with proper license.
However,if you are using earlier versions than I must say that-
It's going to be very very complex and cumbersome development process al-together . (at least for me) .Because why I am telling this is -
The content repository structure of (webcenter site's CA server )is completely different from UCM (webcenter content server and preferred cms for portal).
In previous threads ,lot of things are being discussed about this issue.
In my opinion,you can very well integrate sites content to portal pages via REST API. Documentation is available.
Apart from it, you can also do -
1. Gadgets - You can consume Gadgets from WebCenter Portal, some examples with Google Gadgets are available in OTN.
Hope this info helps you out.