I would create the new dimension in the load script instead of in the chart itself. You can either join the new dimension(s) back onto your original table or keep it as a separate table. Hypothetically, if your original table was called FACTS, then the script for your new table might look something like:
MonthName(FirstOrderDate) as FirstOrderMonth
Min(OrderDate) as FirstOrderDate
Group by IDs
This should create another table keyed on the IDs field that will give you the date of each ID's first order and then a dimension field with the MonthName of that FIrstOrderDate, assuming that your OrderDate field contains quality date data.
For your chart you would use your MarketingChannel field for your rows, the FirstOrderMonth for your columns, and then you can create whatever measures you would like.