That must be the correct assumption. ISL is required to connect more than one switch in the same fabric - which will not be the case for you with the existing set of switches.
Aran had already provided the details - you can connect the host to both existing array and new array with proper zoning.
However, you need to consider upgrading the host software like Navisphere Agent, PowerPath, HBA driver/FW and any other related software. Please ensure that the compatibility is maintained with old and new arrays. You may refer to E-Lab navigator available on Powerlink for checking the support compatibilities.
My 2 cents

Thanks,
Sandip