Are you aware that these are the actual steps?
- Log into wireless router
- Change IP address to a static one on your existing network
- Turn off DHCP server
- Set up wireless parameters however you want to
- Plug existing network into LAN port
- Done. Everyone plugging into other LAN ports or connecting
This has NOTHING to do with Mikrotik networks. If you need help configuring a Linksys router please ask in Linksys forums.