Adding a WYSIWYG editor to a 3rd-party plugin's textarea

Hiya.

I'm using a plugin (Ultimate Member) that builds user profiles. They include the usual fields, but the standard textarea box has no WYSIWYG editor -- which I really need for the site I'm building. (They have an "allow HTML" option for the textarea, but the majority of my users will not know how to use that). I asked the devs if they would add a WYSIWYG editor as a feature, and they said there's currently not enough demand for it.

So...is it possible to enable a WYSIWYG editor to a plugin's textarea box with some custom code? The site I'm building is a non-profit, so the solution needs to be free or very low cost. Any help, suggestions or leads would be greatly appreciated. Thanks!!