я не нашел способ перемещения UDF поля, но я нашел способ клонировать его:
Form myForm = ...;//get Form
myForm.Freeze(true);//freeze form for the flickering problem
Item itemTmp = myForm.Items.Add("NewItemId", BoFormItemTypes.it_COMBO_BOX);
ComboBox comboItem = itemTmp.Specific;
comboItem.ExpandType = BoExpandType.et_DescriptionOnly;
itemTmp.DisplayDesc = true;//display the description of the selected value
comboItem.DataBind.SetBound(true, "OITM", "U_HGR_id");//OITM is the DB table for Items/Articles. U_HGR_id is the UDF field (also column name) that I want to clone to the main window
loadUdfValuesInCombo(comboItem);//a local function that loads the values in the newly created combobox
myForm.Freeze(false);//unfreeze form