If Mail app doesn´t give you enough features for a specific design, the best and most flexible way to build an email signature is to write it in plain html. If you don´t know enough html to accomplish the task, there are also a bunch of free and commercial online services (aka email signature generators) offering visual interfaces to output some kind of working html code.
Word might show you some result in your Word document or in a browser, but it will appear very different in various email clients. That´s what makes it quite demanding to create an email signature. You´ll need to serve quite primitive clients with primitive html, but want to have good looking results on more modern platforms, too. So to say, you can´t just convert word html and therefore need to start from scratch.
Even though Word by itself also has just basic support for html, Microsoft is using Word as the html rendering engine for recent Outlook clients and thus is one of the main reason making an email designer´s life much harder. Fortunately there are various helper frameworks (like Zurb´s foundation for emails) as well as online testing platforms that make it possible to get good cross platform, cross client results with reduced efforts.
I recommend that you have a look at sites like this:
https://www.htmlsig.com/
https://www.zippysig.com/
https://emailsignaturerescue.com/
https://www.exclaimer.com/
https://newoldstamp.com/
https://www.hubspot.com/email-signature-generator
https://www.wisestamp.com/
https://www.sigbop.com/