xaringan can handle putting html into the slides. While I have not tried something as complicated as a flexdashboard, knitting it to html and then putting the html into the presentation may work.
Alternatively, multiple columns can be created with
.pull-right. I don't think there's a way to auto-align rows, but text and pictures naturally show up from top to bottom when the document is knit, and could be manually aligned into rows. The starting template when creating a new
xaringan presentation is quite helpful in terms of the markdown syntax needed to create various effects.
Depending on what you need in the presentation surrounding the flexdashboard, the storyboard pages feature within flexdashboard (http://rmarkdown.rstudio.com/flexdashboard/using.html#storyboards) seems like it might be the easiest option (although I have not personally used this, but it does seem meant for adding a few presentation elements to complement the dashboard).