I noticed some example sentences from Pleco (not sure if they come from there) don't fit at all with characters and definitions. And it affects me a little bit. So I want to add my own sentences, but couldn't find any way to do it. Is it possible to add sentences, and not relying completely on Pleco? If yes, please tell me how to do it.

Thanks in advance

There is a button titled "change" at the end of example sentences. You can click it and choose from different example sentences and also add your own at the bottom. If there is no example for that word yet specified you have to go to the "detail view of a word", for example by clicking the magnifying glass icon while reviewing (top right corner). And then edit the "Example" section.

Have to admit that was skeptical about the example sentences at first, but now find them really valuable since at this point Skritter is the only thing I do in Chinese that isn't using the language in context.

Am sure there is not the critical mass of user demand, but I would love to be able to add example sentences on iOS. I'm happy to help contribute but realistically will never happen if I have to wait until I get to a computer.