I've tried to make a little file, to retain a few more players from my preferred teams. This is where I put the file:
<game dir>\data\db\quboid.ddt
This is it's contents
# Quboid's tweaks.
"RETAIN_PLAYERS" "Derry City"
"RETAIN_PLAYERS" "Liverpool"
"RETAIN_NATION_PLAYERS" "Ireland"
"RETAIN_NATION_PLAYERS" "N. Ireland"
"RETAIN_NATION_PLAYERS" "England"
"RETAIN_NATION_PLAYERS" "Italy"
"RETAIN_NATION_PLAYERS" "Spain"
"RETAIN_NATION_PLAYERS" "Brazil"
"RETAIN_NATION_PLAYERS" "Argentina"
Why doesn't think work? I've started a new game, loading just the Indonesian premier league and a small database to ensure none of these places will have many players auto loaded. However, it appears to have no effect. Liverpool have a decent selection but no more than Chelsea or Man Utd, so I presume it's just due to their reputation. Derry have no one and the countries all have a typical number of players.
In preferences, under extra files > keep players, both harchester and quboid are selected.