vSphere P2V Conversion without converting an iSCSI disk

You can select to not convert that disk in the converter, as simple as that. The iSCSI connection will be usable in the VM and the settings will follow the P2V.

but, IMO, I would recommand to not use an iSCSI disk inside a VM. To be sure before please validate, as when in a VM if the storage is in a iSCSI the quiesce method for snapshot does not work usually.

Virtual machines with independent disks must be powered off before you take a snapshot. Snapshots of powered-on or suspended virtual machines with independent disks are not supported.

You don't convert iSCSI disk -> VMDK (in reality it won't even work), rather you allow converted OS to maintain TCP (...and iSCSI) connectivity.

https://pubs.vmware.com/vsphere-51/index.jsp?topic=%2Fcom.vmware.vsphere.storage.doc%2FGUID-AC22728E-B1B2-48FA-A6D6-62C0045C665F.html

Plus, it's a good idea to allow OS to manage your iSCSI connection, in-guest iSCSI is always slow.

https://community.spiceworks.com/topic/650986-what-is-the-best-place-to-attach-iscsi-esxi-host-or-vm