Is it safe to extend the positive battery cable in any way?

I need to extend the + battery cable on my 1999 Megane Scenic as it is located under the seat and the last owner replaced the terminal clamp leaving it a little short to comfortably access.If it is possible and safe,what is the best way to do it?I cant replace the whole cable as it runs through the bulkhead to a secondary terminal in the engine compartment.Many thanks.