I need to run this:
sudo adduser ...
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Reboot the box in single user mode.
If there are no password prompts, edit sudoers to add yourself.
If there is a password prompt, reboot system from CD/Floppy/USB disk, mount root drive, edit sudoers
If BIOS is locked, remove drive and edit from another system.
If case is locked, pick lock (see Google, YouTube, local bookstore). Then see above.
Use exploit, get root, edit sudoers file.
Perhaps, the design of security can help to clarify your question:
Do you really need to be a sudo? Can you circumvent your problem? There is a beautiful picture here about the process:
Manuals have all kind of nice statements about security, but be realistic! Security is a process that you can not buy from a store. If someone finds a security hole in your system, he may be able to run "sudo commands" without being a "sudo".
Your goal is not necessarily to run "sudo commands" at all. Like the picture showed, people tend to cicumvent security measures. Be pragmatic, and please do not try to head towards a brick wall. Hopefully, the links help.