Can a character add enhancements to named weapons and armor?

Through standard crafting can enhancements be added to named weapons and armor (like Holy Avenger or Banded Mail of Luck) or do those items stay as they are and are unenchantable?

I see text that says ‘usually are preconstructed with exactly the qualities as described here.’ (DMG p219,226) but didn’t see textual evidence otherwise to answer my question.

My suspicion was that they can be enhanced further because non-magical items are included in the DMG such as Mithril Shirt and Dwarven Plate. However, I further noticed in table 7-7 (DMG p220) the non-magical items are not italic making me wonder if this means they are further enhanceable and not the others. The one exception to this is “Mithril Full plate of Speed” where ‘speed’ is on the list of enhancements making me further wonder if I was right.

But I also see no text defining what italic means in that and other similar tables. Further unhelpfully, table 7-2 (DMG p216) shows everything italic.

I realize that “A GM can allow anything they want” could be an answer, but I’m looking for book or FAQ cited text for my answer. Thanks!