Updated with core developer feedback: fixed for Tonne and added tests

src/ifcopenshell-python/ifcopenshell/api/unit/add_conversion_based_unit.py

@@ -32,7 +32,7 @@ def add_conversion_based_unit(
     function. You can choose from one of: inch, foot, yard, mile, square
     inch, square foot, square yard, acre, square mile, cubic inch, cubic
     foot, cubic yard, litre, fluid ounce UK, fluid ounce US, pint UK, pint
-    US, gallon UK, gallon US, degree, ounce, pound, ton UK, ton US, lbf,
+    US, gallon UK, gallon US, degree, ounce, pound, ton UK, ton US, tonne, lbf,
     kip, psi, ksi, minute, hour, day, btu, and fahrenheit.
 
     :param name: A converted name chosen from the list above.

src/ifcopenshell-python/ifcopenshell/util/unit.py

@@ -209,6 +209,7 @@ si_conversions = {
     "pound": 0.454,
     "ton UK": 1016.0469088,
     "ton US": 907.18474,
+    "tonne": 1000.0,
     "lbf": 4.4482216153,
     "kip": 4448.2216153,
     "psi": 6894.7572932,
@@ -253,6 +254,7 @@ imperial_types = {
     "pound": "MASSUNIT",
     "ton UK": "MASSUNIT",
     "ton US": "MASSUNIT",
+    "tonne": "MASSUNIT",
     "lbf": "FORCEUNIT",
     "kip": "FORCEUNIT",
     "psi": "PRESSUREUNIT",
@@ -323,6 +325,7 @@ unit_symbols = {
     "pound": "lb",
     "ton UK": "ton",
     "ton US": "ton",
+    "tonne": "t",
     "lbf": "lbf",
     "kip": "kip",
     "psi": "psi",