In theory, you'd just need to add said riders with an advanced editor like pcm fast editor or lachis editor and put them in your team. The game will just let you play qnd there won't be problems with that except that you won't be able to sign anyone obviously
It's easy really, simply make sure that those young riders that you can find in the cyclist table are allocated in your team (gene_team column or Team only for PCM Fast Editor)
1. Download and install PCM Fast Editor from the PCM 11 Download Section, under "Editors";
2. Open it and select "open database"
3. Now from the menu that opens, find your database. If it's a database, you'll normally find it in Documents/PCM/Mod/Database. If it's a savegame, that will be Documents\PCM\Users\*you*\Database\Solo\*databasename*.
4. Now that you opened it, from the table that will pop-up, find the riders sorting by last name or whatever you'd like. If you want to put them in your team, it's easy: the column team, when it's selected, will open a list with all the teams included in the DB in there: just select your team from there.
5. Now just go on "database" in the upper left corner and click on save database as. It will open a window where you will be able to save your db in the same location as the one you opened it from, but with a different name, so that if anything went wrong you'll be able to revert those changes easily. Choose a name and when you're done click on save.
6. Open PCM and load the savegame/database you've just edited!
Ok then, in that case you'd have to do it manually. When you sign an U23 rider in-game, that rider gets added to dyn_cyclists by the game, completely automatically. But since you are going to add it to your team in a way that is not allowed by the game, you'll need to add them to dyn_cyclists yourself, and furthermore delete them from dyn_young_cyclists, basing yourself off the info in the latter table. Then, as Selwink suggested, add their contracts in dyn_contract_cyclists.
Obviously, a faster way would be to sack some of your riders (in game or via editor) to leave room to those young riders, and sign the riders you need in-game to avoid wasting your team copying and pasting stuff. Then you could even re-add the riders you sacked beforehand easily
