We can create an .XLAM addin with custom functions (UDF). Once we connect Excel to the addin, these UDFs can then be called from another workbook regardless of where the add
If the add-in is loaded, you can call any sub/function with the following code
Sub: Application.Run "YourAddinSub",param1, param2, ...
Function: Application.Run("YourAddinFunction",param1, param2, ...)
If needed, you can qualify the sub. For example: YourAddin.YouAddinSub