Be aware that fitting a sub-baffle over your existing baffle will reduce the available space between the driver and the grille. 25mm is normally enough to allow the cone to move without hitting the grille, although it depends to some extent on the driver. If you have less than this, it might be a good idea to do a 'dry run' first, i.e. screw the sub-baffle in without gluing it.
If this turns out to be problematic, you can always glue a sub-baffle inside the normal baffle. It can be done, but it's more awkward and you'll need to cut the sub-baffle in half to get it in.