Only if you export the model to render it in a third party software. You can replace the decorations with your custom version.
Mecabricks itself only includes elements that have been produced by the LEGO company.

You should add something for local decorations. So for example, i would be able to upload a custom decoration that only i could see, and only i could use

Perhaps you could merge this with another suggestion I've seen.
I've seen a few people want some way to easily change the decoration of something without having to find it in the library and replace it.
So perhaps on the right hand side, (maybe near the colours) you could have a drop down for decorations, and the very last one could be for a custom decoration. If you click on it it would allow you to browse your files for a custom decoration, and allow you to crop it in Mecabricks to make sure it fits on the piece.
You could also limit this to only pieces that have official decorations are able to have custom local decorations.

Have you seen this suggestion?

I have seen it, but it is not something that will happen soon. There are multiple technical issues to this that must be solved first and it is not a priority right now.