Well, the answers depend on how much work you want to do.
As it stands, the two analog lines could be connected to asterisk as two separate channels. But Asterisk wouldn’t know anything about the different phones sharing the same line. It would be no different than your current situation in terms of how these phones share the line (i.e., when two people pickup different phones on the same line, they hear each other, but only one phone call on the PSTN can happen). You couldn’t address them as separate extensions from asterisk.
To address them as separate endpoints, they need separate connections in to Asterisk. You may be able to do this through your phone wiring, if you have access to it. In this case, each line would need to be run separately to asterisk and connected through an FXS port.
Alternatively, if you have ethernet available at the phone locations, you could use an ATA, like a Sipura 2000, and connect these to your asterisk server over ethernet. Your analog phones can then plug into the ATA.
Either of these will get you individual extensions in Asterisk, which can then call each other, transfer calls, etc.
Transferring to a cell phone is a different matter. It can be done (I do it now). For that, I suggest you read up on the Asterisk book and then post a separate question once you know what you want to do.
Hope that helps.