This is a supported feature in Nanacast:

Add upsells using the upsell link on your management screen.

Then on the upsell page, check the option to use custom settings for that upsell. You'll see a box to enter the HTML to be displayed for that upsell.

Below that, you'll see a box to enter code to be included in the head tag. That is where you would put the link to your external CSS file, which should be hosted on your site.