To add a REG_MULTI_SZ multi-line registry value, i can do
reg.exe ADD \"HKLM\\path\\to\\registry\\key\" /v RegistryValue /t REG_MULTI_SZ /d \"abc\\0def\\0\"
Just for reference.
If you just want to insert a new line then you will need to simulate it with a space in the desire empty line. If the space would have an undesired impact in what you are trying to achieve then this post is not useful for you.
reg.exe ADD "HKLM\path\to\registry\key" /v RegistryValue /t REG_MULTI_SZ /d "abc\0 \0def\0"