Is it possible to install this unit in a way that uses the same SSID as my router-modem so I don't have to re-login on my cellphone whenever I move from the router-modem area to the extended area and vice-versa?
can you use the same ssid as your router? Yes you can.
Will it work as you'd expect. Probably not.
Reason why is the newer mesh extenders use the devices (your phone/tablet/etc) roaming protocols to allow devices to switch from router to extender and vice versa.
The WN3000 wasn't a mesh extender and so doesn't have those protocols. Meaning devices are going to be "sticky". In that a device will connect to it or the router and not roam on its own until its signal is pretty much gone. And the problem with this is you can't manually go in and select the better network because they have the same ssid.
