In the case where clipboard sharing is not possible, because there is just a console and no GUI.
If you cannot install Guest additions to share the clipboard, you need to create a script on the host OS that stores the clipboard text in a file in a shared folder. On the guest OS you need to create a script that reads any new file in the shared folder and types the content of the textfile.
old, false answer (may still be useful to others)
In order to enable sharing the clipboard between your guest OS and your host OS, you have to install Guest additions (http://superuser.com/questions/42134/virtualbox-shared-clipboard).
Then you need to run a script on your guest OS that listens for clipboard changes. If you use Windows, you can accomplish something like that using AutoHotkey. The script will type the content of the clipboard as text when the content of the clipboard changes.
For Mac you would also have to use a macro program like Keyboard Maestro or iKey. And for Linux some obscure command line tool that requires an software engineering degree to understand.