If I add another message to an autoresponder series, will clients who are already subscribed get that message?

That depends on the type of message.

If it's a message scheduled to be sent at a number of days from contact subscription, if you have an autoresponder series with 5 daily messages and you update it with a 6th message for the 6th day, the clients who have not yet reached day 6 from subscription will receive the new message on day 6, whereas those who have been subscribed for 10 days and got the previous 5 messages during the first 5 days will no longer get the 6th message in the autoresponder series, since day 6 has already passed.

So, to make sure everyone currently subscribed to the autoresponder series receives a message, irrespective of when they were subscribed, you will want to use a message scheduled at a certain future date. Still, in this case, someone who will subscribe after that date will no longer get that message, but only the messages scheduled at time intervals relative to the date of subscription.

In conclusion, each scenario presents a logical limitation of its own and it is up to you how you choose to go about it.