Custom XF formula for excel add-in

Alt-F11 to open the VBA Editor
Insert a new module
Add your code:
Function XFMemberOffset(referenceValue As Double, memberValue As Double) As Double
Dim Result as double
' Calculate something here

'return the calculated value
XFMemberOffset = Result
End Function

Save your workbook as macro enabled (.xslm)
Alt+Q to close the VBA Editor

In any cell in your workbook, you can now use your custom function: =XFMemberOffset(A1, B1)