Yes this is possible (You can also download models, sprites and maps from
www.esf-world.com)
Modelling:
There are various modelling programs, Maya, 3dstudio Max, Milkshape, Lightwave, just type them into google.com and see which one you like best
Mapping:
If I'm right, the program for mapping is called Valve Hammer, just google it.
Sprites:
You will need Sprite Wizard and Sprite viewer to create your own sprites...
You might also want to check this topic on these forums for more info:
http://forum.esforces.com/showthread.php?t=28814