Tank Viewer
Before you can edit your character you need to exctract the files with Tank Viewer. Therefor you open the Tank Viewer, which should look like this:

Then you open the character file (ending: ".dsparty" can be found in /My Documents/Dungeon Siege/Save)
Now you should see the name of the character file on the left side and the 3 files, "party.gas" "info.gas" and "portrait-0.bmp", listed on the right. Select the your character file on the right and press the extract button in the toolbar. Now you should get a window like this:
Here you select an empty folder and press extract. For each character you are going to exctract you need to make a new folder, so that only the 3 files "party.gas" "info.gas" and "portrait-0.bmp" are in this folder.
Tank Creator
When you have edited the files you have extracted you need to get them back into a ".dsparty" file. To do this we got the Tank Creator, which should look like this:

As sourse you select the folder you extracted your char into. As output you select the ".dsparty" file for your char in the /My Documents/Dungeon Siege/Save folder and press create. Wasn't that easy !!!