I tried on mine (six partitions): no, you can't.
You can stretch the Lion partition by adding another on selecting it and pressing the "+" symbol beneath and sizeing it to 50 Gbytes. When done (if done, depends on data distribution on that partition) you can cancel the new partition pressing the "-" symbol. Youll'have 50 Gbytes free but you can't expand the Data partition into it.
MAYBE (NOT SURE!!!) with apps such as iPartition you can do it but you must startup form another disk using your internal HD in target mode.
Better backup everything, recreate desired partitions from 10.6 installer and restore datas and systems.