Can an upsell add a product with a subscription plan? (Essential Upsell app)

It depends on the offer type.

• Volume discount and product-page offers support subscriptions. The app reads the product's available selling plans and lets the customer choose one. The selected plan is added with the product, and subscription pricing is applied.
• Cross-sell, Product Add-on, and Frequently bought together offers add the selected variant as a regular one-time purchase. These offers do not let the customer choose a subscription plan for the offered product.

If you want a subscription product in a cross-sell or add-on, test it before you promote the offer, because these offers add the product as a one-time purchase.