import existing project (.aia) into new ( which is under developement)

my quesiton is simple I want ot implement = paste another project into one which I'm currently developing, the way so it won't destroy the recent= developing project. Or copy just the block section into recent. and my 2nd question is : how to paste blocks to another screen ( and also some stuff form designer view)



you only can copy complete screens

How to copy screens by  Scott and a video by  Hossein

You might also consider the MIT App Inventor 2 Merger Tool ...  http://appinventor.mit.edu/explore/resources/project-merger-tool-combine-two-app-inventor-projects-one.html   .   It might work better for you but you will only know if you try it.

Make sure you read ALL the instructions on how to use this tool.

I don't know if its weather but I don't get it properly - is it possible to copy one screen to another screen within the same project?

Yes, you can copy one screen to another within the same project.  See the post by Taifun to get instructions on how to do it.  If you do this, we recommend you save an aia of your project somewhere safe in case you do not do the copying correctly and damage your files.