It is simple to inject shellcode (position-independent code). However, I have not seen an example where someone injects position-dependent code (which requires relocations). This project injects a MessageBox into a code cave (a gap of unusued bytes) in an executable file, and applies relocations to the injected code.
The file must already have the API (MessageBoxA) in its Import Address Table. Injecting the import into the IAT of the file may be possible but is incredibly complicated to do. It would require a large amount of adjustments to the other sections and offsets/addresses. At that point, it would be much more logical to write position-independent code.
Demo injection into putty:
