I'm trying to use existing page (selected with pathbrowser from TouchUI) as a popup / modal on my HTL page. Is there a way of adding / injecting HTML from different file using sightly ? It's more about sightly and rendering that html on the page rather then how to build dialogue window.
What you need is to include the content (not the template) within your page, preferrably using data-sly-resource with the path of your page (/content/myapp/content/myPageIwantToDisplayAsModal). this will render the relevant html on your page - remember that in AEM the content knows how to display itself - so as said above you only need to reference the content
Probably you could also utilise the use of selectors not to display the whole html along with <head> section and unneeded <body> tag
so for example:
<div data-sly-resource="${'/content/myapp/content/myPageIwantToDisplayAsModal' @ selectors=['content']}"></div>
for referrence go here https://docs.adobe.com/docs/en/htl/docs/block-statements.html