library(maps)
sm <- map("world", ylim = c(-90, 0), plot = FALSE)

## coords as matrix (including NA for R's old polygon default)
m <- cbind(sm$x, sm$y)

library(rgdal)
## reproject the non-NAs
m[!is.na(sm$x), ] <- project(m[!is.na(sm$x), ],  "+proj=stere +lon_0=155 +lat_0=-35 +ellps=WGS84 +south")
## update the map object
sm$x <- m[,1]
sm$y <- m[,2]
map(sm)