Hi hansT,
To do this, you can use a template. Custom templates can be set via tooltipTemplate property. In the template, you can set various HTML content, including progress bar elements.
Here is an example of how to do this:

<template id="template">
        <smart-progress-bar value="50"></smart-progress-bar>
</template>
<smart-button id="button">Button</smart-button>
<smart-tooltip id="tooltip" selector="button" arrow tooltip-template="template"></smart-tooltip>

Best Regards,
Ivailo Ivanov
Smart HTML Elements Team
https://www.htmlelements.com