Yes, but it would require BIG jumper cables, like #00 or larger fine-strand welding cable. 6 volt systems are very sensitive to voltage drop from undersized cables.
A very short boost with 12 volts to a 6V system won't damage anything. I prefer to do that type of boost with two people, one to make the starting attempt, and another to quickly connect the 12V boost after the cranking attempt is in progress. Be careful not to make any sparks near either battery- - - -make the final connect/disconnect at the ground connection.
Jerry
