I am unaware of any method by which you can return a specific exit code when an MSI terminates.
Unless any other forum member knows of a cunning method to do this, you are going to have to use a somewhat less elegant solution.
What I would propose is that you allocate a specific registry key (or file )  in which to write your exit code, and then have your external applications read this registry key to determine what action needs to be taken if the MSi exits with code 1603.

A somewhat different approach is possible if you have Wise Installation STUDIO or Wise Package STUDIO. In this approach, you test for the Excel running processes using a Wisescript and if any are detected, you can set the exit code when you terminate the wisescript. If no excel processes are detected, you "install" the MSI into the TEMP folder and run it from there, then clean up the temp folder and exit.
The Dragonsoft website has example wisescripts which report running processes using DLL calls, to get you started.

I will suggest the same as EdT said.

Create a Wise script EXE which will check for the exit code and then if it is 1603 change the same to 0 or 3010

Please find a sample script approch

***********************************************************************************************
Set variable PROCEXITCODE to ""
Execute ProgramMSIEXEC /i "%INST%\%PROJECTNAME%.msi" /qb-! /l*v

...since that is the return code indicating that a reboot is required which, in the case of, say, a 1603, almost certainly isn't the case and indeed were it to be acted upon in THIS case (where the only "problem" is that Excel is running) is likely to result in users storming the IT department with flaming torches and pitchforks.