The program Ghkl does electron density skewing (and averaging) in reciprocal space with a spherical mask. This can prove useful in cases where preliminary electron density maps in one crystal form is used to solve another crystal form by the molecular replacement method.
As the program uses a spherical mask, only the (rough) center of the molecule is needed. This saves the time of defining the mask and cutting out the electron density under the mask. In addition, the program works in reciprocal space so there is no need to handle electron density maps.
The program documentation (PDF file) can be downloaded here.