New abilities can be added to specific armor and weapons
While Specific Armors says that "specific suits of armor usually are preconstructed
with exactly [their listed] qualities" (Dungeon Master's Guide 219), and Specific Weapons says that "specific weapons usually are preconstructed with exactly [their listed] qualities" (226), that doesn't prevent specific armors and specific weapons from having new abilities added to them. On Adding New Abilities says
A creator can add new magical abilities to a magic item with no restrictions. The cost to do this is the same as if the item was not magical. Thus, a +1 longsword can be made into a +2 vorpal longsword, with the cost to create it being equal to that of a +2 vorpal sword minus the cost of a +1 sword.
If the item is one that occupies a specific place on a character’s body…, the cost of adding any additional ability to that item increases by 50%. For example, if a character adds the power to confer invisibility to her ring of protection +2, the cost of adding this ability is the same as for creating a ring of invisibility multiplied by 1.5. (288 and emphasis mine)
As the example of making a +2 vorpal sword from a +1 sword makes clear, adding new abilities can be done after a magic item is created. Further, the term creator here means any creature with an appropriate item creation feat rather than, for example, this specific magic item's creator. (The section is part of the larger section Creating Magic Items (282–8) that frequently refers to folks that make magic items generally as creators.) (Also, the Magic Item Compendium repeats and slightly expands this information in Improving Magic Items (233).)
For example, if Yugdoog the Holy wanted to add to his holy avenger (226) (120,630 gp; 4 lbs.) the magic weapon special ability brilliant energy (224) (+4 bonus; 0 lbs.), he'd pay a dude who could upgrade his weapon the difference in price between a +2 cold iron longsword and a +2 brilliant energy cold iron longsword (that's 64,000 gp), drop off the avenger, and in a little over 2 months be able to pick up his brilliant energy holy avenger.
An item name not in italics is a mundane, nonmagical item. Thus it's simply an oversight that the mithral full plate of speed (220) (26,500; 25 lbs.) is not in italics—an oversight that continues even in the premium edition Dungeon Master's Guide (2012). The presence or absence of italics typically has no impact on whether or not an item can be made more magical or made magical, respectively. (The system of using italics to differentiate magic items and spells from the game's other material dates back to at least Advanced Dungeons & Dragons (1977) and continued in Second Edition (1989). Third Edition's use of italics for magic item and spells is a touchstone for those who'd played previous editions; that the core rules never explain this is reprehensible.)