Can I use KY Jelly to lubricate the HYALO GYN Vaginal Hydrating Gel applicator?

It is not recommended that you mix products. If you are not backfilling or overfilling the HYALO GYN applicator, there should always be excess product available for external use. 

This should be enough product to add a small amount to the external portion of the applicators provided with the product. It depends on how much you will need for your comfort.