If you are sending the webinar invite to all of the leads that are in that nurture stream, you can just add the invite in the stream. If you drag it to the top of the stream and set the availability, they will receive that invite next time instead of the next piece of content and then 7 days later continue down the stream.
If this applies to all leads, you could make sure you have your email communication limits set accordingly. If not, pausing the stream may not be a bad idea?