Pre-populating workflow initiation form with list item data

Hi,

I have an initiation form in my workflow that updates project timelines. It would have been nice if I can pre-populate it with the dates from the list item and let the user change what neede to be changed.

I came across articles about this like http://generation12.wordpress.com/2009/03/25/enhanced-workflow-initiation-forms-in-spd-part-ii/ but the form to replace is an ASPX file. I looked at the form I used in the workflow and it's not an ASPX but an XSN file.

How can I make this work? I can only use SPD.