The Championship outfit are already well-stocked at the back, with potential recruitment focused in other areas.
Zola would have to move someone out in order to make room for Hall - who was released by the Hornets at the end of last season as his contract came to a close.
He was allowed to train with the club over the summer in an effort to keep himself fit, with injuries having restricted him to just 22 appearances last term.
No club was found and Hall has returned to Watford after taking in a recent trial spell at Millwall.
Zola is unsure what the immediate future holds for the 32-year-old centre-half, with it not always possible to turn interest into something tangible.
He told the Watford Observer: "Fitz came back this morning to train with us. I don't know what is happening with Millwall."
Pressed on whether there is still hope for Hall at Vicarage Road, Zola added: "It is a question that I cannot answer right now. We have a lot of players and a lot of competition in that position.
"We have a high value and a high consideration of Fitz. None of us have ever doubted his qualities on or off the pitch. The only problem is that in that position we had several players and now we have Nyron Nosworthy coming back from injury.
"I see it as a very difficult thing to happen, although it is a pity because we know how good Fitz can be."