Add to Existing Contact from Mail.app to Contacts.app
This is a quick look at this,. which means it can either be refined or there is a better answer.

I will assume you know how to create an automator service. So here is an image showing the setup.



Applescript 1:

on run {input, parameters}
    set bigList to {}
    tell application "Contacts"

        repeat with i from 1 to number of items in input
            set this_item to item i of input
            copy name of this_item to end of bigList
        end repeat
    end tell
    return bigList
end run


Applescript 2:

on run {input, parameters}
    tell application "Mail"
        set eachMessage to item 1 of (get selection)
        set theExtractAddress to extract address from (sender of eachMessage)
    end tell
    tell application "Contacts"
        set thisPerson to first person whose name is item 1 of input

        make new email at end of emails of thisPerson with properties {label:"home", value:theExtractAddress}
        save
    end tell

end run


It will ask for permission to access the Addressbook the first time you run it.
And is a bit slow to return the search results. but worked in my test.