Here is a workaround for this:
Use word to compose a one-word greeting, i.e.: Hello, in whatever font you choose. Copy and paste that into Outlook when you are starting to compose an email, and the rest of the body will have that custom font.
Caveats:
When pasting, you have to long-press on the text field to paste the text. It won't work if you use the mobile clipboard because it will merge the formats.
Also, this works as long as that text is the last thing you copied, so you would have to repeat the process every time you need to start a new email if you've copied some other text. So, consider saving that word document and have it ready to use.