You can't add offerings to an existing plan on Subkit.
Subkit doesn't allow you to add offerings to an active plan. To add physical products, content or sessions to a plan, you'll need to create a new plan. You can deactivate the existing plan and set up a new one with the offerings you want to include. Just so you know, you can always reactivate a plan if you need to.
When creating a subscription plan, you'll need to confirm all the offerings you intend to include in the plan. Once you click the activate button, you can't update to add more offerings. However, you can edit other details like plan name, description, free trial and subscriber notes.
To create a new subscription plan, read this guide.