I think the F1 teams do trials for the pit crew. You don't need to be a skilled mechanic after all, just quick, dextrous and precise. So they select from the people who could be in the travelling team anyway - and have another job - and then they do the pit crewing as an added bit of work. Which requires lots of extra practice throughout the year. Redbull nailed 3 stops that were under 2 seconds this season! You don't get that without days and days of running drills - critiquing them at a ludicrously detailed level of precision - and then doing it all over again.