If i want sweet cider can I just add more sugar than a yeast that can only take like 5% apv and then use a 6% yeast (or one that would be correct to eat enough to equal the amount needed) to carbonate?
You won't be able to control yeast's sugar consumption that precisely. Even the cheapest bread yeast from the grocery
Store can survive at 9-12% or more.
The difference needed to carbonate is about 0.25%. There's no way.
You can let it ferment for a while. When you hit your desired gravity, kill off the yeast. This won't help if you want to carb it up though.
The second option is to back sweeten it with something that is not fermentable such as an artificial sweetner.
You could look into pasteurizing. The general concept is: let your cider ferment completely, add a bunch of sugar to sweeten, let it carb for a few weeks, give the bottles a hot water bath to kill the yeast.
If you try this, get some good instructions and make sure you know EXACTLY what you're doing. I've never done it myself. You're basically guessing how much carb you have, and you need to be 100% SURE that you can kill the yeast, otherwise... BOOM.