Pack HU in another HU

Hi

We have the need at goods receipt to pack incoming materials (in an inbound delivery) in existing HUs which contain the same material.

We are getting the error that "The HU is already put away".

Is there any possibility to directly pack materials in existing HUs?

Alternatively, is there any way of transfering stock from an existing HU to another HU, without unpacking first and not nesting HUs? We have Z fields assigned to HUs and we can't lose them when moved to the other HU.

Thanks