Waaaaw, I didn't think that my previous post, Create e custom Workflow using SharePoint Designer, was going to be that famous :-), because a lot of questions already came up.
1. Can we directly put it in the calendar
Well in the action where you send the email you can add an action to add it directly in a calendar. You could even connect this calendar with MS Outlook.
2. Can you change the pages created by SharePoint Designer
Well even that is possible. In our workflow, two pages are created. A page to allow a user to start the workflow manually and the page where the Human Resource Manager needs to complete his task.
Open these pages and customize them to your own needs.
Keep the questions coming: firstname.lastname@example.org