@Patrick30. You are correct that you can add any prompt you want using html/javascript. You might want to look at how the handlebars template system works.
Here is some documentation: Using ODK-X XLSX Converter — ODK-X Docs
NOTE: By providing a customPromptTypes.js file in your form directory, you can define Backbone views that extend the base prompts.
Here is a previous post on example creating a custom prompt type: Problem with customPrompt in survey - #3 by AlbertoB