I am not sure how you would do that. 
One way could be to see how you would do it with Hugo. If you render your static flexdashboard to html, you may be able to insert it in static
See this question in Hugo forum: https://discourse.gohugo.io/t/what-is-a-better-way-to-include-a-full-external-html-page-and-its-resources-into-blog/9287
Let's not that the flexdashaboard would be included as a completly different layout from your blog.
In blogdown directly, or to insert inside the layout, this will give you hint on what is going on:
However, I am really not sure what is possible because flexdahboard is a full layout framework with header, footer and all, you can't easily include that in another website without some tuning.
Maybe someone will have example of this somewhere, but i don't know one yet.