MongoDB does not support any other datum than WGS84 (and WGS84 is de facto standard these days). If your data uses different reference ellipsoid then I could only advise converting it to WGS84 before storing it in MongoDB if you would like to use geospatial features in MongoDB.
Here is somehow a related topic converting WGS72 to WGS84: Converting between coordinates system WGS 74 and WGS 84
If this does not meet your app requirements then check out PostGIS.