We are trying to find a way to integrate xsl-fo support in robohelp classic 2019. Is there a procedure to follow thus we can support xslfo in robohelp for publishing and html production. What we are thinking from our part is an xml import -> (transform) -> xsl-fo ready -> generate different sources(pdf,html, etc.)
That's more a use-case for Adobe FrameMaker.
FrameMaker 2019 comes with Saxon Enterprise Edition and allows you to "plug in" pre- and post-processing XSLTs as part of a custom "structured application".
E.g., you could build a structured application where you plug in a pre-processing XSLT that transforms your incoming XML to the XML you want to have for editing/publishing (e.g. DITA). In this scenario, you could just open your XML in FrameMaker, let FrameMaker transform it to DITA, get it rendered based on your designed template, and then use FrameMaker's built-in publishing engine to publish to PDF, Responsive HTML5, etc. Or you could develop your own DITA Open Toolkit output type and publish through this route. Or run your own output XSLT.