You've got several hundred foot pounds of force with a very small cross-section impacting you. Do you really want a piece of aluminum deforming in to your chest?
The point with ceramics is they crush with a minimal of "splatter", and are usually wrapped with kevlar to prevent the escape of secondary projectiles. Rifle rounds which are defeated by metal tend to splatter with great energy. SAPI plates contain several layers of different material (can't recall the composition off the top of my head), but the purpose of the design is to force the slowing and deformation of a rifle projectile prior to it getting to your body.
Adding extra plates - aluminum, steel, or otherwise, will not protect you from rifle rounds one bit, unless those plates are proven to defeat the projectile standalone. It is not safe to assume that it will protect you any better against handgun rounds higher than the vest is already rated for.
Vests are shot with a clay backing so they can measure the amount of deformation. In order to be certified to NIJ standards, vests have to defeat projectiles while not exceeding a certain MAXIMUM amount of clay deformation behind it. This represents and indicates the amount of blunt force trauma sent in to your body. Vests which allow too much energy to be transferred over too small of an area will not receive certification to NIJ standards, as those vests could allow potentially lethal force to enter the body, even though the projectile itself is stopped.
On the matter of putting in steel plates; I've shot a lot at and through steel plates, including 223, 308, 7.62x54R, 7x57 mauser, 8x57 mauser, 7.5 swiss, 300 Win Mag, and 50 BMG over the years. 308 FMJ will readily penetrate 3/8" mild steel, while harder steel might stop it in it's tracks. Most stuff you get your hands on, that you could reasonably fit in to a trauma pouch, will not readily defeat a rifle round. Even if it does, I wouldn't want my face anywhere CLOSE to the plate if a rifle round hits it.
There are very few plates certified to stop repeated hits by 7.62 NATO / 308, but there are some on the commercial market beside the SAPI plates used by the military.
If you are in a line of work where bad guys with guns are a possibility, and you are concerned about the possibility of small arms fire, why take chances with some bit of scrap metal and duct tape from the hardware store when there are items perfectly suited to the task and readily available commercially?
NOTE: This link is to a 1.2 MB PDF document identifying NIJ test standards for certification.
NIJ Body Armor Certification Guidelines
This is a link to a listing of all currently certified makes and models of body armor which have been tested and found to be compliant. (Be patient, takes a minute for the list to appear.)
Ballistic CPL Listing