Answer
Custom menu items are placed within a dedicated block at the top of the menu, but you can control their order within that block: when setting up the menu item, there's a 'Menu Item Order' section where you can move items up and down relative to each other. This is a system-wide setting (not profile-specific), so reordering it changes the position for every user who sees that custom menu item — but you can't interleave a custom item into the middle of IFS's own standard menu options, only reorder among your own custom items.