Hello David,
You can add an hidden field which you can insert user id of next participant.
You can fetch datas from your global list with following code:
WorkflowGen.My.Integration.GlobalLists gl = new WorkflowGen.My.Integration.GlobalLists();
System.Data.DataTable GLOBAL_LIST_DATA;
GLOBAL_LIST_DATA = gl.fetchList("YOUR_GLOBAL_LIST", "COLUMN_NAME1,COLUMN_NAME2", "en-US","value","Equals",YOUR_DROPDOWNLIST.SelectedValue,true);
HIDDEN_MANAGER_ID.Text = GLOBAL_LIST_DATA.Rows[0]["COLUMN_NAME1"].ToString();
HIDDEN_SUPERVISOR_ID.Text = GLOBAL_LIST_DATA.Rows[0]["COLUMN_NAME2"].ToString();
You can find an example here : Form Designer: How to bind a text box field to a global list
Quentin