how to cast CDT Type to record type ?

You can do it like this

recordType!Student(
    recordType!Student.fields.name: local!student.name
)

Just make sure to replace recordType! with your actual record properties. 

Using cast() function.

cast(
  'recordType!{xx-xx-xx-xx-xxx}Employee',
  'type!{urn:com:appian:types:HROB}Employee_CDT'(
    id: 1,
    name: "Abc",
    department: "IT",
    salary: 123,
    age: 25
  )
)