not sure I understand you entirely, so I will be brief.
you want to add data to a specific row in a data grid? (i.e. multiple lines of data)
If so, there are a number of ways to do this. you could have two datagrids that are linked by an event i.e.CurrentCellChanged (update the other datagrids dataset, and control them seperatly). Or, you could add another table to your data set, that your current datagrid is bound to. Then, setup a relationship between the two tables (parent > child). You can then drill into a specific row and add detail to the new table in your dataset. ???? not sure if that helps.