To be able to add an external digital signature, you need to edit the wse script that Wise creates when creating exe-files. And in order to be able to edit the wse script, you must force Wise to create the wse script permanently in the same folder together with the wsi file. By default, the wse script is generated only temporarily at compile time and deleted afterwards.

Go to Installation Expert - Release Definition - Prerequisites. Add a dummy prerequisite (for instance notepad.exe). Then you are able to push the Edit Script button. Edit the wse file and select Project Settings - Distribution - Digital Signature and select "Add a digital signature externally". If you cannot see this option, close the wse file, note where it is stored (in the same directory as the .wsi file), open WiseScript Editor from the Start Menu and open the wse script by selecting File - Open. Now the menu should be visible.

Don't forget to manually remove the dummy prerequisite from within the wse script (you should ignore that it still shows on the prerequisites page in the .wsi file).

After this, you need to keep (version handle) both your .wsi file and .wse file together.They have to be in the same directory when compiling.