According to the doc of shinyDashboard::valueBox() you can provide an icon that you can build with shiny::iconand built in in this function you found fontawesome but also glyphicon. see ?shiny::icon
You may find the icon you want in glyphicon.
If none is ok with you, I guess there is an advanced use possible using css, js and modified HTML. It is more web development than R dev, but I am not sure if it is easy or not.
The code of shiny::icon could help understand how to use another JS icon library if you really need too.
Hope it helps.