I would create a package level (or scope it to the data flow) variable of type int and set to zero. The code in the Script Transformation will use the Input0_ProcessInputRow procedure to increment your variable and your Identity Column. Something like this should do the trick:

Option Strict Off

Imports System

Imports System.Data

Imports System.Math

Imports Microsoft.SqlServer.Dts.Pipeline.Wrapper

Imports Microsoft.SqlServer.Dts.Runtime.Wrapper

Public Class ScriptMain

Inherits UserComponent

Public Overrides Sub Input0_ProcessInputRow(ByVal Row As Input0Buffer)

MyBase.Input0_ProcessInputRow(Row)

Dim vars As IDTSVariables90

Dim Count As Integer

Me.VariableDispenser.LockOneForRead("PipelineRows", vars)

Count = vars(0).Value

vars.Unlock()

Me.VariableDispenser.LockOneForWrite("PipelineRows", vars)

vars(0).Value = Count + 1

Row.Ident = Count + 1

vars.Unlock()

End Sub

End Class

Keep in mind that you'll want to set your identity column (Ident in my exampe) to ReadWrite on the Input Columns page of the Script Transformation editor.