It sounds like you need to do some research. Here are the MSDN links to the objects you are working with:
First of all, a DataSet is a collection of DataTables. When you say you have a column in your DataSet, you probably really mean that you have a DataTable in your DataSet and that DataTable has the columns.
Now, in order to "convert" the data from your ArrayList you will have to loop through the ArrayList and add each item into the DataTable that is contained in your DataSet. You can create the loop using a For Each statement. Here is a sample (It's in VB.Net, but you should be able to get the idea and convert to C# pretty easily):
Dim myDataSet As New DataSet
Dim myDataTable As New DataTable
Dim myArrayList As New ArrayList
For Each obj As Object In myArrayList
Dim rowNew As DataRow = myDataTable.NewRow
rowNew("Column1") = obj.ToString
If you have other columns in your DataTable, you'll have to fill those as well. Also, depending upon the data type of your columns and what data you are filling your ArrayList with you may have to convert the objects in your ArrayList into the proper data type.
As for getting the total to appear in your crystal report, you will have to edit the report file itself to include your new field. You may have to setup groups if you want group totals or you may have to setup a page or report footer. You'll want to do some research either by google or through articles here on Code Project to research how to do that.
Hope this helps.