This from Microsoft support :
Automation Add-ins and the Function Wizard
Each Automation Add-in has its own category in the Excel Function Wizard. The category name is the ProgID for the Add-in; you cannot specify a different category name for Automation Add-in functions. Additionally, there is no way to specify function descriptions, argument descriptions, or help for Automation Add-in functions in the Function Wizard.
This article also mentions that other addin types take precedence in the formula list.
My prefered approach is to write a VBA addin that wraps up the functionality that you expose in your current addin. I Do this by implementing my code in a COM DLL (That supports IDTExtensibility2 so it loads as a COM addin), and then access other methods on that object via very thin vba wrappers:
Public Function AddinInMethod ( param As String ) As Variant
Dim oAdd As Object
On Error GoTo Err
Set oAdd = Application.COMAddIns.item("MyProgID").Object
AddInMethod = oAdd.AddInMethod(param)
Exit Function
Err:
AddinMethod = "#" & Err.Description
End Function
This works well for me since all my code is C++.
Since you are writing C# code, you might be better off implementing your add in as a C# VSTO add-in. I haven't done this personally, but it looks like there are lots of nice framework features that you can take advantage of as this type of Addin (though you are limited to newer versions of Office).