You CAN NOT use a panelboard as a junction box.
NEC 110-3(b) Installation and Use. Listed or labeled equipment shall be installed and used in accordance with any instructions included in the listing or labeling.
UL-67, to which panelboards are tested listed and labeled, does not list a panelboard to be used in this way.
Here is an interesting read on this subject, that includes a response directly from UL.
http://www.isa-home-inspections.com/articles/tecspk08.pdf
