The maximum skill level of walk-ons is limited by your tier. A tier I team will occasionally land an A-rated walk-on, but if so the player's grade is only marginally above the A/B borderline. Tier II teams may land some B-rated walk-ons, while Tier III teams should land only C-rated walk-ons and lower. Walk-ons primarily serve to give you backup players and fill positions that you don't have enough players in. In order to have the best chance of getting good walk-ons, you will want to have a legal roster before walk-ons are added.