You can download a full Truetype font onto most ZPL printers directly using the ^DY command. This is the recommended way to do this unless the font is bigger than the printer's usable storage. You would have to check the printer's User Guide to find out how much storage your printer has. This is not usually an issue unless you are working with Asian fonts or older printer models. In that case, you can use the Font Wizard that comes with Zebra Setup Utility to make the font smaller by selecting only the characters you need. This converts it to a FNT font as well. The direct method will allow you to call the font as a TTF or OTF.
~DYE:MyFont,B,T,XXSIZE,,...binary data of full font
Details: https://www.zebra.com/content/dam/zebra/manuals/printers/common/programming/zpl-zbi2-pm-en.pdf?page=164
Like Jason mentions, Please remember to make sure your font is licensed to you for this use.