Appending arrays of fixed arrays of bytes.

Hello. I've been trying to write some code that appends arrays of
fixed arrays of bytes, but I have yet to succeed. I haven't found any
examples of this, so I'm not sure if I'm doing it wrong or if I'm
doing it right but there's a bug. Before I mess with this any further,
can someone tell me what the correct way of appending arrays of arrays
of a type is?

Thanks,

Christian