Thankyou for using Microsoft Q&A platform and thanks for posting your question here.
As I understand your question, you are trying to know the syntax of using 'cast' or 'convert' function in copy activity of ADF pipeline where your source dataset is pointing to Servicenow. Please let me know if that is not the correct understanding.
'Cast' or 'Convert' functions are not available in ADF . However, if the source supports it, then you can use the query option in source setting of copy activity to write down the query with the same syntax which you use in service now to run it on the service now endpoint.
Revert back in comments if you face any error while debugging the pipeline with this approach. Hope it helps. Please accept the answer by clicking on accept answer button. Thankyou.