Making it into a global UNA template is beyond my knowledge :) But if you are just trying to use the HTML format/style on lets say a single custom UNA page, in that case, it should be possible using it by creating a new page from una studio, adding an html block, and pasting your html in there. Depending on how the format of the HTML is, you might have to adjust it i little bit to make fit/look good on the rest of the UNA template.