What you need to do is go to multiplayer, create a session, pick your settings then let the first map you chose load and play.

Press T to type as usual, then, type this:

/admin /changemap then put something like cor1c_c -thats the Knightfall level.

it would look like

/admin /changemap cor1c_c

then that map will load. you can use this admin command to run other standard maps withiot creating a new session.

there are more admin commands designed to run on servers, google them.

some of the career maps's code is:

cor1c_c
fel1c_c
geo1c_c
kas2c_c
myg1c_c
uta1c_c

I don't know them all, but you can guess, g is for gcw, so maybe try nab2g_g

other online players can join and play with you, they may crash or not be able to spawn since the campaign maps are auto assign randomly.

to swap a player, use /admin /swap and a number(1 is admin)
/admin /swap 2 would force the 2nd (or 1st) person to join to swap sides.

hope this is fun!

The code for the campaign maps is as follows:

cor1c_c
dea1g_c
fel1c_c
geo1c_c (training level)
hot1g_c
kam1g_c
kas2c_c
mus1g_c
myg1c_c
nab2g_c
pol1g_c
tan1g_c
uta1c_c
yav1g_c

also space campaign too like space coruscant