Can I create hacks with this project

It is technically possible to create hacks with the EoS decomp in its current state, but it is not much better than binary hacking. There are still hard-coded memory addresses within the assembly code and data, so if you edit any C code or insert/remove lines of assembly, code will shift without updating these hard-coded addresses, breaking the ROM. We are currently working to replace these hard-coded addresses with symbols that the linker will properly update when code shifts. When all hard-coded address are removed, the project is considered "shiftable", and at that point it is possible to update C code and insert new lines of assembly to create hacks without breaking the ROM.

If you want to create EoS hacks at this time, the premier binary hacking tool is SkyTemple. Alternatively, you can consider using the PMD Red Rescue Team decomp, which is shiftable.