Can you add all the missions separately?

Yes, it is possible, but there is one thing. And it lies in the fact that no one knows where the mission starts from. Of course, we can say that the mission begins after the inscription of the completion of the last mission. But I don't think that's a good idea.

There is a lot of problems in making IL's for this game in-particular, because it's open-world. And there is a problem, that you can just unlock stuff that you were not supposed to early, like weapons, upgrades and skills. There also would be a problem with that you can cheat skills that you were not supposed to have at that point in time and it would be really hard to track "legit" from the cheated one.

You could say, that you can make a unique save for just IL's and if you don't use it — your run will not be allowed on the board, but for some reason, Ubi decided to put your Uplay ID into a savefile, so you can't share your saves with anyone. They fixed it in later installments of the series.

So, until there is mod that allows you to just enable and spawn on a mission (Like leaked debug-version from Xbox), there will be no IL's for this game.

it is always an option to just have zero limits on equipment and gear for the IL's :) Go ahead and get the best gear, best skills, and steamroll the levels as fast as the game allows!