Doubt: add a boderline map

Hello,

I got a map through the plot function.

Code:

map <-plot(proj$A_EMmeanByROC_mergedAlgo_mergedRun_mergedData,
main= "", xlab ="x.coords", ylab="y.coords", cex.axis=caxis)

However I needed to add a border line in my study area to become more
perceptible (Figure 1).

I tried the border(), rect () functions  but it gave error.

So, I tried to create a border line

Code:
nc.sids <- readShapePoly("PoligonoSM.shp")

plot(nc.sids, asp=1, main="")

nc.border <- unionSpatialPolygons(nc.sids, rep(1, nrow(nc.sids)))
plot(nc.border, asp=1, main="")


But I am having a lot of difficulties in building the script.

How can cross the information of the 2 plots?

https://www.dropbox.com/sh/y5by1sxjcvdl6rh/AAAIF4srkChW2FxeEfFG4d_ta?dl=0
Thanks.

        [[alternative HTML version deleted]]