Final answer:
John Witherspoon was an active clergyman when he signed the Declaration of Independence, also signing the Articles of Confederation and participating in the Constitutional Convention.
Step-by-step explanation:
John Witherspoon's occupation when he added his signature onto the Declaration of Independence was an active clergyman. He served as an influential figure in the revolutionary activities leading to American independence. Witherspoon was not only a signer of the Declaration of Independence, but he also signed the Articles of Confederation and attended the Constitutional Convention in New Jersey (1787) which ratified the Federal Constitution, demonstrating his active participation in the founding of the United States.