An additional prayer that the Marian Movement of Priest recites after the Fatima prayer is
“Come Holy Spirit. come by means of the powerful intercession of the Blessed Virgin Mary, your well beloved spouse.”
So, yes, you may pray additional prayers between decades. Just don’t add too many. I personally only pray the two. Any more would be too many.
I have prayed with groups that simply sing “Ave, Ave, Ave Regina” after the Fatima prayer, as a way to keep the rosary at a meditative pace.