R - plot_brmap ( state border )

I made a map of Brazilian cities using plot_brmap() and I want to show the state border.

But I don't know how to do it. I have been trying to fill the mapa2_data (state map) with transparent color and plot with mapa_data (city map). But it's not working.

What I have been trying:

library(brazilmaps)
library(maptools)
library(RColorBrewer)

    # Definindo paleta de cores ----
    createColors  <- colorRampPalette(c("yellow","red","darkgreen"))

    # Definindo cidades e estados ----
    cidades_sp_mg <- get_brmap(geo = "City", geo.filter = list(State = c(31,35))
                               ,class = "SpatialPolygonsDataFrame")

    estados_sp_mg <- get_brmap(geo = "State", geo.filter = list(State = c(31,35))
                               ,class = "SpatialPolygonsDataFrame")


    # Criando Mapa - Cooxupé e Cocapec ----
    load(file="dados/data.Rdata")

    # Agrupa por razao social e municipio, conta quantas coop. cada municipio tem
    dScore_data <- data %>% group_by(cod_municipio_ibge) %>% count()


    # Gera mapa das cidades
    mapa_data <- plot_brmap(cidades_sp_mg, 
                                       data_to_join = dScore_data, 
                                       join_by = c("City" = "cod_municipio_ibge"),
                                       var = "n"
                                       )

    dScore_data$code_state <- as.numeric(dScore_datac$code_state)
    mapa2_data <- plot_brmap(estados_sp_mg, 
                                       data_to_join = dScore_data, 
                                       join_by = c("State" = "code_state"),
                                       var = "n"
    )


    MinhasCores_data  <- createColors(4)

    png("dados/Cities.png", width = 800, height = 600) 

    print(mapa_data) +
      scale_fill_gradientn(colours=MinhasCores_data,name="Qtd.", na.value = "lightgray") + 
      labs(title="Cities",caption = "Font") + 
      theme(legend.title = element_text(face="bold"),title = element_text(face="bold"),
            plot.title = element_text(hjust = 0.5,size=20),
            plot.caption = element_text(hjust = 0.5,size = 10)) 
+ plot(mapa2_data)

    dev.off()