For 1st season, find euroseededteams.csv at \Game\dlc\dlc_FootballCompEng\dlc\FootballCompEng\data and replace ids with teams you want to include in competitions.
For next seasons, inside compdata folder you have settings.txt.
Search (ctrl+F): info_special_team_id,393
copy/paste the whole line underneath and replace the id by id of added teams.
there are two places where you have to do that, the other one is a little further down the page and is for europa league