Missions shown in Campaign.def are different in the actual game.
 

Using Pffeditor, I opened localres.pff in Delta Force Xtreme. There was a file called campaign.def. Its contents were as follows-
//Campaign def file
// title - ID of str table entry for title of campaign
// - These ID strings are specified inside Game.rc
// briefing - ID of str table entry for campaign briefing
// - These ID strings are specified inside Game.rc
// image - name of image file to display for that campaign
// depend - name of campaign(s) that must be completed before this campaign is available
// mission - [mission file] {dependency} -> mission followed by mission(s) that must be completed before it becomes available in campaign
//
// This campaign menu system also allows for pictures associated with
// the missions to appear in the menus. Those missions are to be named
// according to: MISSION_NAME.TGA (Note: the loadscreens the game supports
// are not to be confused with this. Those loadscreens are called: MISSION_NAME.PCX

complete_overlay complete.tga

begin peru
title ID_CAMPNAME_PERU
briefing ID_CAMPBRIEF_PERU
image peru.tga
mission C2M01_B.bms //Insurrection
//mission C2M02_Y.bms //**Masquarade
mission C2M03_P.bms C2M01_B.bms //Flood
mission C2M04_R.bms C2M03_P.bms //Weatherman
mission C2M04a_X.bms C2M04_R.bms //Peruvian Gold
//mission C2M04b_B.bms //**White Rabbit
mission C2M05_Z.bms C2M04a_X.bms //Bad Habit
mission C2M05a_Y.bms C2M05_Z.bms //Reckless Path
//mission C2M05a_Z.bms C2M01a_Y.bms //**Swift Justice
mission C2M06_Z.bms C2M05a_Y.bms //Headhunter
end

begin chad
title ID_CAMPNAME_CHAD
briefing ID_CAMPBRIEF_CHAD
image chad.tga
// depend peru
//mission C3M00a_X.bms //**Angel Eyes
mission C3M00b_B.bms //Thrown Rotor
//mission C3M00c_Y.bms C3M00b_B.bms //**Three Kings
mission C3M01_Y.bms C3M00b_B.bms //Exodus
//mission C3M02_X.bms C3M01_Y.bms //**Carrion
//mission C3M03_B.bms C3M02_X.bms //**Snake Eyes
mission C3M04_P.bms C3M01_Y.bms //Dust to Dust
mission C3M05_R.bms C3M04_P.bms //Metal Hammer
mission C3M06_X.bms C3M05_R.bms //Full Stop
end

begin navoya
title ID_CAMPNAME_NOVAYA
briefing ID_CAMPBRIEF_NOVAYA
image navoya.tga
// depend chad
mission C6M01_Z.bms //Birds of Prey
mission C6M02_P.bms C6M01_Z.bms //Chain Lightning
mission C6M03_B.bms C6M02_P.bms //Rapid Strike
//mission C6M04_R.bms C6M03_B.bms //**Scorpion
//mission C6M05_Z.bms C6M04_R.bms //**Snow Blind
//mission C6M06_P.bms C6M05_Z.bms //**Fallen Angels
mission C6M07_R.bms C6M03_B.bms //Whirlwind
mission C6M07a_R.bms C6M07_R.bms //Frostbite
mission C6M08_B.bms C6M07a_R.bms //Storm Chaser
mission C6M09_Y.bms C6M08_B.bms //Pandora's Box
mission C6M10_X.bms C6M09_Y.bms //Snow Fox
end

Now, whichever mission has 2 stars before it is not present in the game. Also, if I opened the localres.pff file in black hawk down,can i add some files to it. I have some problem with the missions of the game, and i saw a file named original stock maps of bhd here. So, i thought whether i could replace the files in the pff file and replace them with the downloaded ones.

You should be able to import missions (and files) into the .pff with no problem.