When you export your project from Creator to Eclipse, there might be certain pre/post-build commands which get generated, like the one you have in your screenshot. To add your own custom command, just add it after the existing command seperated by a semicolon.
So, let's say you have to run a python script, "sample.py", you can add the command as follows:
<existing command>;python sample.py;
That worked great, thank you!