wiring in a Perko Switch

Want to add in second battery with a Perko switch. Would like to use the existing battery cable in the boat. Question is, can I cut off the connector that's on there and crimp on a new one? Or solder? I need to change the end at the battery to an "i" type connector to connect it to the Perko. Also, what type of connector would I use to attach the ground to the two negative terminals? The directions and diagram with the Perko switch seem simple, but I'm just not sure what to do with regards to the connections. Thanks.