epsg90013 to epsg4326

You can transform them using library code, via pixel space. First make sure that active map is in EPSG 3785 (forget about 90913). E.g. just use default OSM map. Then convert your projected coordinates to pixels of some zoom using linear calculation, like they were map pixel coordinates. Note that with bigger zoom accuracy is better. Then use map service mapPosToWgs method. My code:

// some constants
double xmax = 20037508.34; // ymax is same for EPSG:3785
int basezoom=20;

// test coordinate conversion
double x = -742990.597843593; // in projected
double y = 7056832.45410695;

// convert to pixels on some zoom
double xpix=(1+(x/xmax)) * Float11.pow(2,basezoom-1)*256;
double ypix=(1+(y/xmax)) * Float11.pow(2,basezoom-1)*256;
		
// convert to WGS
Point wp = map.getMap().mapPosToWgs(new MapPos((int) xpix,(int) ypix,basezoom));
System.out.println(wp);

I got result as: 53.39732S, 6.674398W which is quite close to your numbers.

Finally you can use the WGS point in library API to add points to map. This will convert it internally back to pixel coordinates, but this should not too much overhead.