Thanks to @juba, @rsc and @SlowLearner I've found out, that the installation of gpclib was still missing to be able to give the gpclibPermit. With this done, fortifying sf
using a specified region is not problem anymore. Using the explanation from ggplot2/wiki I am able to transfer all data fields of the original shapefile into a plotting-friendly dataframe. The latter finally works as was intendet for plotting the shapefile in R. Here is the final code with the actual workingDir-variable content left out:
require("rgdal") # requires sp, will use proj.4 if installed
require("maptools")
require("ggplot2")
require("plyr")
workingDir <- ""
sf <- readOGR(dsn=workingDir, layer="BK50_Ausschnitt005")
sf@data$id <- rownames(sf@data)
sf.points <- fortify(sf, region="id")
sf.df <- join(sf.points, sf@data, by="id")
ggplot(sf.df,aes(x=long, y=lat, fill=NFK)) + coord_equal() + geom_polygon(colour="black", size=0.1, aes(group=group))