Hi, I'm one of the Invoice Ninja developers.
By clicking 'Add Product' next to the buttons, you should be able to add things to an invoice with tasks. By setting the task's client, saving the record, and then choosing 'More actions,' you can also add tasks to an open invoice.
We're working on a new version of the software (https://www.subscriptionflow.com) and aim to expand on this part.

Helpful article: https://www.odoo.com/forum/subscription-7/mass-invoicing-of-subscriptions-158134

This is more a technical question rather than a subscription question. you can try by adding the subscription name to the 'source_document' field of the invoice. If that doesn't work, you'll probably need to update directly in the database.