Use struct2cell() on the old structure, and fieldnames() . Then insert a new cell of values for the new field at the top of the cell of data, and insert the new field name at the top of the list. Now use cell2struct() to create the new structure in the order you want.
This is the only way to control the order of fields when you insert a new field into a structure.
I would suggest to you, however, that it is easier to allow the new field to be put at the end of the struct fields, and then use orderfields() to change the field orders.