Yes, to make your mod:
1) Download the Freelancer SDK (all files pre-decoded) and install
2) Open up the mbases.ini file
3) Search for Li01_01_base
4) Find lines looking like this:
offers_missions = true
mission_type = DestroyMission, 0.000000, 0.112387, 50
Well change it to say somethign like
offers_missions = true
mission_type = DestroyMission, 0.000000, 80,100
Save the file, and that is YOUR mod!!! Congrats, tougher missions from manhatten bar (dependant upon the faction u choose missions from). Well - glad thats sorted.
Oh wait, you didn't want to increase the difficulty - well, sorry, without being SPECIFIC about what you want to do there is NO WAY ON EARTH we can help you - its FAR to general
So work out what you want to know, then try looking in the stickied thread that boasts about how its got it all (includeing download links to the tools you'll be needing!).
There we go, have a GOOD LOOK in there before askin any questions, cause the chances are ITS BEEN ANSWERED. Then if its not there - check the TUTORIALS forum - where most things are kept too. Finally - USE THE SEARCH BUTTON.
Remeber, to see OLD threads (which are FULL of info), you can select the length of time topics go back by using the box in top right corner of your screen in the forums.
Be warned, no-one likes to type out somethign that has been covered 10 times already because someone hasn't been bothered to look, so do NOT just ask how to do something, actually try to find it first. You have to put effort into modding, so don't be expecting others to pile effort in for you because you don't search!
Any questions about how to do something after that, and people will be only to happy to answer, or if you don't understand either