Not only is the training a good idea, it's essential! But first you have to establish the schools and hire the teachers. Then you end up with a bunch of inexperienced construction workers. Folks who are more likely to get injured, die, or change professions.
That's why we need people who have already dedicated their professional life to the trade to get things started. Right now America doesn't have enough.
There are highly skilled building trades people all over the world. Many have a hard time working in the United States because the government recognizes college degrees far more than trades. I would like to see the government change this policy for a few years.
Don't know the specifics on how you would qualify the trades people, I'll leave that to better minds than mine.
The advantage for born Americans is that once this project is successful there will be a huge availability of low cost apartments, duplexes, and retail spaces that can be purchased through a federal housing program.
Most Unions in the US have apprenticeships to receive training. That said, I believe that medium to low skill labor will be phased out during our lifetime. Automation will replace much of the workforce and it is important to be able to pivot to the next needed skills. I believe that we should provide more education in the skills that will be needed for the coming economy. Software engineering, Cyber Security, Social Media influencing, and Search Engine Optimization. Notice that none of these careers will require someone to live in a specific location. A person with these skills can live anywhere in the world.