diff --git a/configure.py b/configure.py
index 47568a55..931adc5c 100644
--- a/configure.py
+++ b/configure.py
@@ -4988,8 +4988,29 @@ config.libs = [
             Object(Matching, "data/model/int_yos_wheel.c",),
         ],
     ),
+    # custom TUs to be injected into foresta.rel (for modding).
+    # anything added here should also go into config.add_objects[1].
+    # example:
+    #    under 'game_custom' lib: Object(Matching, "src/path/to/my/custom/c_file.c"),
+    #    under config.add_obejcts[1]: ""src/path/to/my/custom/c_file.c""
+    {
+        "lib": "game_custom",
+        "cflags": cflags_foresta,
+        "objects": [
+            # Object(Matching, "src/path/to/my/custom/c_file.c"),
+        ],
+    },
 ]
 
+config.add_objects = {
+    # module_id => object names
+    # module_id 0 is DOL
+    # module_id 1 is foresta.rel
+    1: [
+        # "src/path/to/my/custom/c_file.c",
+    ],
+}
+
 # Define our custom asset processing scripts
 config.custom_build_rules = [
     {
diff --git a/tools/project.py b/tools/project.py
index d5d5652d..702639fe 100644
--- a/tools/project.py
+++ b/tools/project.py
@@ -1107,6 +1107,10 @@ def generate_build_ninja(
                         },
                         module_link_step,
                     )
+                # Add custom TUs into the REL
+                if config.non_matching:
+                    for unit_name in config.add_objects.get(module["module_id"], []):
+                        add_unit({"object": None, "name": unit_name, "autogenerated": False}, module_link_step)
                 link_steps.append(module_link_step)
         n.newline()