Thank you for contacting Syncfusion support.
In order to use Syncfusion control inside script template, we suggest you to render the control from script so that the control will be rendered in Dialog Template. We have prepared sample as per your requirement to add DropDownList with TreeView in Grid Dialog template.
For your reference check the following demo link: http://asp.syncfusion.com/demos/web/grid/dialogonlocaldata.aspx
Please let us know if you have any other queries.