parted /dev/sda resizepart 1 Y 30GiB && resize2fs /dev/sda1

In short

# Convert to vdi
VBoxManage clonehd "vm.vmdk" "vm2.vdi" --format vdi

# Resize
VBoxManage modifyhd "vm2.vdi" --resize 20480

# Convert to vmdk
VBoxManage clonehd "vm2.vdi" "vm.vmdk" --format vmdk

Then start the VM and extend the partition via gparted.
See: step 3 from derek molloy

Source

• security-helpzone
• derek molloy