The Data Delivery Extension or DDE can be installed on a machine other then the ArcIMS machine. But if the DDE was installed on a machine other then where ArcIMS is installed, the Authorization Wizard incorporated in the ArcIMS Post Installation will not be available to register and authorize it. This article includes the procedure authorize DDE for use.
If the ArcGIS Server of ArcSDE is installed, you may already have a keycodes file. Instead of overwriting the existing file, open it with Notepad and paste the text from the new .ECP file into it and save it to update the file for DDE.