I made a vfat disk image of a usb using dd ...
I hope you mean "I made a disk image of a vfat partition using dd
If that's what you meant, you can't do it like that.
You cannot add space to your partition with dd
You need to use a partition manager eg. fdisk
, because this correctly writes the changes to the partition table.
Create and format the partition you want the files to be on.
Mount the disk image file with the loopback option ( read man mount
for the details)
Copy the files across to the newly created and formatted partition.