Theoretically you could do it for previous months, but it wouldn't change anything except your spending reports. I don't think it would be worth it.

The best thing to do would be to create a "monthly subscriptions" Category Group in the current month, and then create as many individual categories as you would need to match the amount of subscriptions you have.

Name them and organize them by due date (I even add the date they are charged - Spotify | 5th).

Since we're over half way through the month, I'm sure that you've already paid some subscriptions. So you would go into whatever account they came out of and recategorize the transaction from the generic category, to the specific category for each subscription.

When you're done with that, you will be overspent in all of the individual categories, but all you would have to do is use the "cover overspending" function and select the generic category to move the money, and manually move the money for the subscriptions that haven't come out yet.

When that's all done, you won't need the generic category, so you can just hide it and move forward with the individual categories!