New Page as www-data (not virtual user)

[YourDoom]

Hey,
I need ImageMagick on my Server and i want create a user with www-data privileges.

www-data can't execute 'convert ...' (exec('convert ...')) on php, www-data can.
Sudo don't work
How can i make a virtual user to a real user?

Thank you for your support

Best regards
(and sorry for my bad english)