Hmmm - odd that it won't let you use or apply the car filter in Practice sessions - I wonder why that is?
I have to be honest and say I don't think I have ever used just Practice in the game at all.
If I wanted to be able to do the overtaking loads of cars on Nords. here is what I would do -
I would select - RACE EVENT - then on the menu for race settings I would select YES - for Practice Session 1 and Practice Session 2, Also increase the number of OPPONENTS to say 23, you dont have to Race the event you can keep restarting Practice if you like I think.
1. Set the AI lower so they are slower, if you want, but this is less fun.
2. On the car selection Screen in the middle at the bottom is the CAR FILTER button - select this - the TOP LEFT of the next screen is where you click to SWITCH CAR FILTER ON - then click CREATE FILTER - create a filter - using this filter can be a bit difficult at first as its not all that Intuitive - (play around with it and see what you get in game is best).
Proceed and see what you get on Nords. - RESTART SESSION - at your convenience to keep yourself in Practice Session 1, but since you said YES to Practice Session 2 its not that important anyway, also if you click CONTINUE instead of going into qualify and Race through to EVENT RESULTS - the last screen gives you a BUTTON either side at the bottom - one is MAIN MENU the other is RESTART SESSION which will take you back to Practice Session 1 and you can do it all over again.
It may not be the answer you were looking for but I think it will give you more than enough of what you wanted. Please let me know because although I have just been oin game to work it all out I have not gone all the way through to Practice on the track - if I had I may well have got sidetracked into racing for a few hours and forgotten to come backand post.
Yes I am addicted to Sim Racing I get in game and it just takes over - It could be worse though, I could be addicted to something really bad, like crack, herion, alcohol, Gran Tourismo or even the X-factor !!!!