I do not see any easy way to accomplish this task using Mail or even Automator.
There are ways to accomplish this but it requires multiple complex steps and would only be beneficial if you had a rather large number of contacts you wished to import in this manner and needed to repeat the process many times. It would involve scripting and or programming. In most cases the effort exceeds the time it would take to just manually add each contact one at time. It would have to be hundreds of people in the list before I even attempted such automation.
Basic overview of the process might be to forward the email and copy the TO:, CC:, BCC: lines containing all the names and email addresses and paste them into a text file. Then once they are in a plain text file there are a million different ways to reformat the file to either CSV or vCard format. I could search and replace using a programming editor or I could use UNIX commands in Terminal to manipulate the text (SED, AWK, TRIM, CUT, XARGS, etc.). I could use a more powerful scripting language such as Python or Ruby to do it. But again, this can be a lot of work for a one-off operation that you might not be repeating. If it's something you really need to do often then it might be worth coding a custom App in Swift to do it.
Bottom line, it's possible to do most anything but is it worth the time and effort?