Is the traditional supplication, "My sin is great, and Your pardon is great, so combine my sin with Your pardon, O Most Generous," sound, and can one supplicate with it?

The supplication mentioned, "My sin is great, and Your forgiveness is vast, so combine my sin with Your forgiveness, O Most Generous," was narrated from one of the pious worshipers. Scholars mention it to illustrate the awe of the predecessors and their good opinion of Allah.

In summary: The supplications authentically reported from the Prophet (peace be upon him) suffice and provide more blessing and soundness than this supplication. There is no prohibition from supplicating with it occasionally, but it is better to say, "So encompass my sin with Your forgiveness."