Contributing content to Wikipedia rather than linking to the content externally is always preferable, but I'm not quite sure of what you're proposing. If you'd like to write a section for the income tax article on the US income tax system specifically, there's nothing stoping you; administrators like myself aren't here to arbitrate permissions (beyond simple vandalism and other disruptive edits). I do think your proposal sounds like original research with a distinct bias inherent to it; for that reason I think you should read Wikipedia:No original research and Wikipedia:Neutral point of view carefully before making any further edits. Your contributions are welcome and appreciated, as long as they're neutral and encyclopaedic in nature.
While I'm here, I should welcome you to Wikipedia! I hope you like the place and decide to stay. (Follow that welcome link for a good place to start learning about editing, as well as read up on our poliices and guidelines). If you've any further questions, don't hesitate to ask. Cheers, -- Hadal 03:56, 8 Jan 2005 (UTC)