You have to have access to unity, unfortunately. It requires rebuilding the asset bundle that contains the font, but before that it requires actually generating a textmesh-pro-style font, which looks at a ttf or otf font and then generates edge data images which allow for infinite zooming and scaling rather than being bitmap-based.
After a font asset is created in that way, it has to be applied to the various prefabs in the gui unity project, and then the asset bundle is built. So it's not a super quick process, but it's not the slowest thing in the world, either -- for us, anyway. If you don't already have unity and whatnot set up, it would be a particular pain.
Typically going through all that process is a bit wasteful anyhow -- it's a lot easier to just pop open photoshop or gimp, take a screenshot and blank out existing text with content-aware fill or similar, and then start inserting mockups directly. Super duper fast way to get a feel for things. In the cases of fonts that must be bought, we've even done the thing where we type a phrase into the font preview website, then screenshot that, remove the white, flip the black to white, and then use that in a mockup to see how the font feels in context. Otherwise we'd be buying every font under the sun before finding the perfect one.