This quote is not quite right and changes the meaning of what Junius says here. Junius distinguishes adnasci (something like growth) and infundi (infusion). But what he then defines is adnasci, not infusion as the above states. The text is actually saying that something is added to natural law in a way that accrues or grows onto it.
So one qualification here is that Cocceius argues that man was created for a covenant with God, but the covenant is not natural to him. Adam does not immediately stand in this relation at creation. We might say Cocceius is a "third way" here. Van Asselt argues that the covenant of works is "synthetically not analytically" related to creation (Van Asselt, Federal Theology, 260).