I have first generation Unitary Patent; will it automatically extend to additional states that join the Unitary Patent System?

The territorial coverage of a given generation of Unitary Patents stays the same for their entire lifetime, irrespective of any subsequent ratifications of the UPCA after the date of registration of unitary effect.  

Thus, there will be no extension of the coverage of Unitary Patents to other Member States that ratify the UPCA after the registration of unitary effect by the EPO.