If it's a GNU C/C++ project or a Clang C/C++ project, you should be able to go to Build -> (GNU|Clang) C/C++ Options, go to the Compile tab, and add that "-m32" flag to Other Options.
It varies for other project types, but if you have no entry under the Build menu for the compiler options and your compiler honors the CFLAGS environment variable, you can set that variable for the build by going to Project -> Properties, click on the Tools tab, select the Build tool, click on the Advanced button, and then click on "Set Environment Variable" in the dialog that pops up, and then set it there.