Its not too difficult expanding the map north or west, but in addition to the basic graphics work on the *.tga files it is necessary to "shift" the coordinates of all the existing objects and bridges to suit the location of the new map origin. Any new land area created will still need to have objects added to populate it.
A good start point is to read the stuff here
http://www.sas1946.com/main/index.php/board,121.0.html
and especially here
http://www.sas1946.com/main/index.ph...c,33463.0.html
There is an Actors Cut and Shift tool, which as its says, is used to shift the coordinates of objects or to crop down object sets, if say a map is reduced in size.