Classic Outlook
You add text-transform: uppercase; in your HTML source.
<span style="text-transform: uppercase;">((DisplayName))</span>
Example: The 'John Doe' is show as 'JOHN DOE' in his email signature.
New Outlook
ALLCAPS in NEW OUTLOOK
If you are using New Outlook, you may notice that the 'text-transform' is not working properly. This happens because New Outlook strips or ignores certain CSS, including text-transform. That’s why uppercase/lowercase transformations won’t render properly in HTML email signatures or templates.
Instead you can use a liquid filter to make the field ALLCAPS.
{{ 'SoMe TeXt' | upcase }}