Hi,

right now there is no such option. There are several requests on uservoice where you could throw your votes at:

Need to customize the workflow Start form using nintex forms. this option is not available. – Customer Feedback for Nint… 

start form – Customer Feedback for Nintex 

Initiation Form – Customer Feedback for Nintex 

Modifying the start form would really be helpful. As a workaround right now you could only create a list and modify its form. After entering the data in the form a workflow kicks off, working with the data entered in the list. But this is not always a good solution and results in using additional workflows / lists.

Best regards

Enrico