Is it possible to have 1 global user with which I can access all domain ftp's? At the moment on a presentation environment it's quite annoying to create a new user each time I want to upload a project to our it.
No. Maybe you can create a linux system user manually and add this user as member to all web[ID] groups and set the home directory of this user to /home/www or /var/www depending on your website root. But i dont have tested that.
Doesn't this give any right problems with template engines like SMARTY? Because the user/group will be different from the custom user/group for the website itself.
No, i dont think so. The user you create is memeber of the webX group and as long as the webX group has write permissions to the files, there shall be no problem.