This edits a critical game file. Carry on at your own risk.

Locate the config.cfg file in the COD2 directory - a focused file search will find it easily. Open it using Notepad and locate the "seta mis_01" string. Set the value of that string and all other "seta_mis_" lines following it to 10 or higher to unlock those stages on Veteran difficulty. It would appear like:

////////////////////////////////////
seta mis_01 "13"
seta mis_02 "13"
seta mis_03 "13"
seta mis_04 "13"
seta mis_05 "13"
seta mis_06 "13"
seta mis_07 "13"
seta mis_08 "13"
seta mis_09 "13"
seta mis_10 "13"
/////////////////////////////////////