connecting SIP phones without an adaptor and provisioning as separate extensions?

I have five yealink SIP phones (T20s, T46gs and W52s) that I am currently using with a local 3cx server. If I open an account with VOIPo, can I just plug my SIP phones into my regular router and provision those for my VOIPo account or do I need an adaptor (even though I am only using SIP phones)? And if I can do that, can I provision them as separate extensions so that I can still call directly from one extension to another in addition to making and receiving calls from the outside via VOIPo? In other words, can I connect all my SIP phones to the VOIPo service and provision them as separate extensions?

You can if you get the Voipo hosted PBX for $20 month, or use your own PBX and have the phones connected to it, then the PBX to Voipo phone service.