How do I make an html template work with UNA?

Is it a UNA template but sold there? As far i know it has to be UNA compatible and since it is not available for download/install from UNA store products, it will have to be manually uploaded to your UNA folder site.

It's just a plain html template, but how can I turn it into a UNA compatible template?

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.

Mixing your own html into your una to look the way you want is hard but doable sample site work (images added is temporary and will be replaced with HD photos and right size)