AFAIK KS-NS 300 is a IP PBX which support SIP phones. Technically you should be able to use a 8845 if the phone is on MPP firmware.
You must refer guides released by Panasonic regarding third party sip phones.
Kindly verify the existing firmware. if its on enterprise versions, follow the guide shared by @Jonathan Schulenberg to convert the phone to MPP before you try with Panasonic. Conversion is not free, cost involved.
How to configure them on Panasonic, you must refer Panasonic guides. You wont be able to find this info on Cisco.
Personally, I would not make any attempt to do so.
For a start, the phones will need to be converted to 3PCC/MPP firmware. Next, users will need to get Service Contract to download firmware because the integration could run into a brick wall of bugs (which requires a Service Contract to download firmware).
Bugs discovered will need to have Service Contract to report them to TAC. If the bugs are never seen before, it will take several years for the bugs to get fix.