The iRedAdmin feature "Fetch mails via POP3" under "Service Control" doesn't mean the fetching of 3rd party mail accounts, but rather one of the deliver method from iredmail solution itself!?
Yes, you are right.
That means there is no preperation that I can use to fetch my emails from other provider to the "local" virtual ldap user?
I think the example showed in fetchmail manual page should work for you:
poll pop.provider.net proto pop3 port 3111
user "jsmith" with pass "secret1" is "smith" here
user jones with pass "secret2" is "jjones" here keep
It maps remote user 'jones' to local user 'jjones', so it will deliver this mail to local user 'jjones', right?
If so, you have to setup a LDA (Local Deliver Agent) to deliver these mails, such as 'deliver' program which provided by dovecot package (it's included in iRedMail).
I may be wrong, because i'm not familiar with fetchmail program.