The problem is not in iReport (or, more correctly, jasperReports), but it is not in JDBC either.
From JDBC driver I get "correct" data - they are really in CP852. I mean, JDBC driver is not converting to Unicode, so my problem is not related with your link.
What I need is just conversion of (some) string data columns, which came from database, in iReport, so that they are displayed in Windows CP1250.
Strangely, solution like:
new String($F{nPaket}.getBytes("cp852"), "Cp1250")
where nPaket is field name, does'nt work - it converts, but wrong!