According to Wikipedia, to play PSO offline, you need a "Silver Account". I have a original XBOX, how can I play offline?

borrow an account from a friend or use a 2 month trial

I guess that's one way

So, is there any other way?

Do you have a modded xbox? There are offline patch xbe's. Easiest way I can suggest to you, however there are others which require a bit more work.