In a BizTalk map, you can add scripting functoids without connecting them to anything. They can contain functions, as shown below.

Note 1: You must add a constant for each variable, so if you have 3 parameters, then add three constants values on the “Functoid Inputs” tab.

Note 2: They will have a warning icon on them, indicating no inputs or outputs.

Now, anywhere else in the map, you can add a Scripting Functoid, and simply call the shared C# function from above:

A better approach is probably have to have a C# Map.Helpers class/assembly where you have all the reusable functions. Then you can also re-use them across different maps.