Do you mind me asking when you say "the free email generator is not a solution" what do you mean? Within Hubspot, if you go to your account settings, then Email > Signature, you will see a dialog box where you can either have a rich text format setup or HTML. Using either of those should allow you to add the social media icons horizontally. Also, am I safe to believe you're after adding email signatures to the sales emails or to the marketing? I think both allow that option, however, the aforementioned is for the sales side of things.