The first answer is yes. You ATA will plug into you network via a switch or router. Depending on your ATA you will most likely have a web interface to put in the connection settings for your VOIP service (ie. host address, username, password) and the connected analogue phone will work using that VOIP connection.
You second question gets complicated. Depending on your service provider for your normal telephone number you should be able to takes calls to your normal and immediately forward them to you VOIP number. I'm sure most normal phone service providers have this as an option. There is probably a star code (eg, *21 for Telstra Australia) where you enter the star code and then the VOIP number, eg *21 5551234. Your service providers website or customer support will help you with setting that up. That will cost you each time to forward the call though to VOIP.
The other more complex way is to port you normal number to you VOIP provider. This gets difficult depending on the owners of the number. I would ask you VOIP provider if they can have you number ported to them. This way you normal number can become your VOIP number.
