Can I add another local c disk or replace it, I need more memory on my W500 Lenovo?

Replacing might work. You will have to move over the contents of the disk to the new one.

Do you need more Memory or do you need more storage space?
Memory normally refers to RAM which is NOT the Hard Drive.
IF your C: drive is too small you can replace it with a larger one, but you will have to reinstall the Operating System and all the drivers.
If all you need is more Memory RAM then that is easily done by adding different size RAM chips.