ggmap with Gauss Krueger coordinates

Question

I am using R for doing a map of groundwater levels which means I have some measurements at some coordinates in the Gauss-Krueger system and I interpolated between these measurements. I want to plot this interpolated distribution map on top of a satellite image of that area. Using ggmap, I can only retrieve the map for the corresponding Lon-Lat coordinates, but I actually would like to plot the whole thing in the GK system. Is it possible to do this, or alternatively, convert the axes of the satellite map somehow? I thought about converting the GK coordinates to Lon-Lat, but I am not sure about the accuracy of that, since the area of interest is pretty small, maybe 300x300 meters.