@chintan97 you can overlay the below jsp to customise the popup, the data for the new field will be save under the metadata node of the workflow instance. So you should be able to read this data from workflow object.
/libs/cq/gui/components/authoring/workflow/startworkflow/startworkflow.jsp
But what is the purpose of this new field, Can this not be added into the dialog participant step rather then overlaying the OOTB component ?