GRASS will be very helpful to you since it uses a [computational region][1] to work on :

> In computations,

> * raster input maps are automatically cropped/padded and rescaled (using nearest neighbour resampling) to match the current region in order to produce the output raster map or to query values. 

> * **Raster output maps have their bounds and resolution equal to those of the current computational region**. 

> * Vector maps are always considered completely. 

So, using GRASS, you will define the region according to the existing raster

       g.region rast=myExistingRaster


(Adding *-p* option to the command will display raster properties)

And then, run the command to rasterize your vector layer :
      
       v.to.rast


More info on [v.to.rast command][2] is available online.


  [1]: http://grasswiki.osgeo.org/wiki/Computational_region
  [2]: http://grass.osgeo.org/grass64/manuals/v.to.rast.html