I'm using Transmit.app (in Mac) to sftp to my Ubuntu server. Is there any way to gain superuser permission so that I can remotely write any file?
Why not using
There is a good client for osx -> click
It's generally not recommended that you allow direct root remote access. If you enable direct root login via SSH, anyone could attempt to brute-force your root password, at which point they can do whatever they like with your server.
That said, if you're set on doing this, you should set up public key login, as it's more secure than password-based login. That page includes instructions on how to set this up for root. Once you have that configured, you can configure your SFTP client to login via key rather than password.