Can I create hacks with this project

Tl;dr: yes, it is possible to create hacks with the EoS decomp.

This decomp is "shiftable", meaning that all pointers use symbols that are swapped out for the proper pointer addresses by the linker when building the project. It is therefore possible to update, insert, or remove C, assembly code, and data without breaking the ROM.

Caveats

C structs should be treated with caution unless you are sure that all usages of a particular struct have been taken out of ASM and decompiled to C. If a C struct is still used within raw ASM, you can only add fields in place of padding, as this avoids shifting other struct fields or increasing the size of the struct.

Integrations

Symbol names in the decomp are synced to and from pmdsky-debug.

The decomp is not currently compatible with popular EoS binary hacking tools like SkyTemple, though there are plans to support SkyTemple integration in the future. For the time being, any tools that edit the ROM's file system (e.g., data files or cutscene scripting) can be transferred to the file system in the decomp by extracting the modified ROM's file system. ASM patches do not transfer easily at this time, and must be ported manually.