On cliking add new post redirect user to a custom page first

Hmm – I have to wonder why you don’t just have the selection that you would have on the first page be in a metabox in the add new page – and maybe make it appear top right ?

Given that the implication is that they could change the prefilled data anyway?

Failing that, a possiblity may be to
1) add a separate menu item for your special page. (and/or remove add new )
2) submit of special page calls/includes “add new” file if special fields prefilled in _POST or _GET variable
3) metabox function will detect the prefilled variables and display them as you wish.

Kind of like this – it has a “a new link” from a settings page.

http://webdesign.anmari.com/2643/initial-content-for-a-new-page-or-post-wordpress-pleasant-surprise/