I had done this too when trying to see if I could get more top USA prospects to go to colleges, If you use the Loan feature in the extra_config file you can loan players out for longer than season so theoretically you could have prospects with set college transfers in like 2027 and have them loaned out to another team until they're 16 and their loan ends and they end up in the USNTDP. Its kinda clunky with younger players though so I probably wouldn't go younger than the players you already have.
I did, however, find out that its only players under 50 World Rep who want to sign with colleges and it seems like they sign even if they have 200 Home and Current Rep so that kinda solved my problem there. Colleges also just assign lower rep players who were born nearby to their squads which I believe U16 clubs do too, which makes it easy to fill out colleges since you can just make a pool of low rep players born from the Cities the colleges are based in and they'll get assigned to a college, the only problem is this applies to all colleges so I ended up purging all the unplayable college leagues. Some cities were really weird though (for example all the players from Grand Forks, ND in my testing seemed to never get assigned to any NCAA teams, and they often all got assigned to the same ACHA team in Colorado, just ended up replacing them with more players from Minnesota since they seemed to get assigned to North Dakota just fine)
after the above changes here are a couple of the worst teams in the NCAA from 2028-29 and they are pretty filled up: