It's not a real combobox; it's made by a Pane with image and label (without selecting state) and a listview in a form (selecting state)
Otherwise, you can change the alignment on this sub, on pos.GetField
B4X:
Private Sub hBoxSetAligment(obj As Object)
Dim pos As JavaObject
pos.InitializeStatic("javafx.geometry.Pos")
pos = pos.GetField("CENTER")
Dim j As JavaObject = obj
j.RunMethod("setAlignment", Array(pos))
End Sub