LabVIEW does not support nibbles as data type, the shortest is U8. Thus you probably want to split each byte (U8) into two bytes, ignoring the 4 high bits in each.
This is easy to do using masks. Take each U8 and "AND" it with x0F (00001111) to get half, then rotate the original U8 by 4 vits and repeat the masking for the other half.
Attached is a simplified example (LV 7.0) that probably does what you need. Modify as required.