The answers above are less than helpful.
First, you can register anything as a trademark, provided that it acts as a trademark.
If, for example, you wanted to register a mark in the US, then you are going to have be offering goods or services in commerce using the mark.
So the question is not "can I register X as trademark?", the question is "can I register X as a trademark for the business of trading in Y goods or services."
What sort of business do you conduct or intend to conduct using the domain name as a mark?
And perhaps a more relevant question is in what jurisdiction would you like to register the mark?