This is a current limitation for customization of Enterprise PDM. These files are located in the WfIcons.dll in C:\Program Files/SolidWorks Enterprise PDM folder. It would be a great request since larger workflows could use more icons. In the 8 years using the product, this is one limitation that has been there since the begineeing. ~Lou
The only way I know of to add more icons to a .dll file is to use the freeware program Resource Hacker. It allows you to edit dll files to add resources to it. You can also create new .dll files which is what I used to do to put all the icons into for custom AutoCAD menus we used at work.
You can find it at: http://www.angusj.com/resourcehacker/ The one caveat is always make a copy of the orginal .dll file before you start modifying in case you screw it up while editing.
Retrieving data ...