Thanks @S Goutham 
Yes you were right there is no such option to add date picker in g_modal. But is it possible to add other reference field based on choice field selected in service workspace.
For example:
if choice field have 3 choice values and if value 1 is selected then based on that some reference field should be made mandatory and available in pop up and should be hidden if other choice value selected to show other mandatory field.

Thanks in advance.
Prabhavathy