Sims cancel actions

Is there a way, i can put new actions to the sim's queue without making the sim cancel it's current action?

I think you mean when they do actions out of their own free will..
When you 'force' an action on them, it will cancel the one they have decided for themselves to do.
I'm not sure if there's a way to turn it off, I kinda prefer them doing what I want anyway

No no, let's say i order my sim to eat some food. while he is eating then, i command him to go to the toilet. then he stops eating and instantly goes to the loo.

Oh that's odd, mine just complete whatever it is they're doing before they move on to the next action.
Does this happen in all save files or just the one family?

Hmm. I'm playing only one family of 4 sims atm. But i will try to find out, if a new family will act the same. Thank you for answering btw ;-)

I've had this issue a few times myself, so you're not the only one.