IAR Breakpoint in Externally Build Program

I am using IAR Embedded Workbench for ARM 8.40. I am trying to debug an existing .elf file that was written using Notepad++ and built using a makefile.

I am able to import the existing executable into IAR following these steps: Debugging an externally built executable file | IAR Systems

I am able to "download and debug" the program using IAR and my PEmicro Multilink. The debugger initially stops at main() like I configured in the debugger options menu. I am able to resume and pause the debugger and the correct C file will pop up in the IDE. So the debugger is able to find the source files that are elsewhere in my PC that I used to build the elf file.

My question is how do I set breakpoints myself before I start the debugger? I am tried Project > Add Files and added the files I want to debug. However when I try to flash the HW, I get build errors from the newly added C files.

Ideally I want to continue using Notepad++ and makefile to edit and build the source code. I just want to be able to set breakpoints and debug using IAR. Is my only option to create a new IAR project and import all the source files, linker, etc?