One sim, all careers possible?

I think it is possible. What you have to consider though is switching career branches. Let's say you completed the writing career in the author branch and want to switch to the journalist branch. You can't just quit the job and then select the other branch. That would have been too simple and straightforward.
The only two ways I know is by getting the corresponding degree in university or by using mods. Mccc lets you reset the career branch.