Sorting multiple separate league competitions into one table by points per game

[BKmind]

I'm not 100% sure if this is even possible, but I am looking at creating a system where several competitions (with different numbers of teams/games played) run in parallel. At the end of the season the standings of each league would be brought into one massive table with all teams, and to account for discrepancies between games played it would be sorted by points per game. The idea is that from this table the top 64 teams would then qualify for a knockout tournament, so instead of taking an equal number of teams from each league, it takes the best 64 out of all of the leagues. My first thought was creating a hidden stage in a parent competition, but I'm having trouble figuring it out.