Can I connect a firewire 400 harddrive to a router with only ethernet ports so I can access the drive wirelessly from an iMac and Macbook Pro?
No. I don't know of a FW to Ethernet adapter. If such a thing existed, it would cost more than a NAS drive anyway.
Does your router have a USB port? If it does, you could move the drive to a USB enclosure and access the drive that way.
Retrieving data ...