Um, actually I did a while back , and someone corrected me in the comments . This directly conflicts with what’s in the WSS v3 SDK that states you “must use activities provided” for SharePoint Designer when comparing it to developing workflows with Visual Studio 2005 . OK, so we now know that’s not true… well how do you build these?
Carlos Segura Sanz has posted two entries on his blog on how to add custom workflow activities to SharePoint Designer… a good read!
» Add custom workflow activities to SharePoint Designer (Part 1) [ En espanol! ] » Add custom workflow activities to SharePoint Designer (Part 2) [ En espanol! ]