No, there is no formal integration like that in place currently. You can just copy-paste from another site, or give out the URL link to your players, or keep the other site open in another browser tab. They don't integrate yet, and I could see where someone would want that, but I try to keep in mind that Roll20 is trying to reduce programming bloat / learning curve / potential slowdowns, and keep this site flexible for all types of RPG's. In some cases it may be best to just use another website for whatever that website is good for (Campaign management, making maps, etc), and use Roll20 as just your tabletop that can benefit from the output or collaboration of other websites without formally integrating line by line.