Adding files after USB creation, rewritable filesystem

I don't know if this is related with persistence, but after 0.3.0 I am unable to add files on USB-created. For example, I added some personal data and files which I used to copy to the system after installation.

I also have a personal distro which automatically copies, through preseed, some files located on custom USB-created directories to predefined locations after installation.

However, after 0.3.0 I am unable to do so. I had to downgrade from 0.3.3 to 0.2.67 in order to get it working.

is it possible to get this feature back? How can I work around this?