Best option: Backup the data on the ext3 drives, add them to your host server, create the new data store, restore the data from backup.
If retaining the drives' formatting is a primary concern, you might be better off placing those drives into another server and using iSCSI to access that 2nd server.
If retaining the use of those hard drives is the primary concern and you have two other servers or decent computers that could be used temporarily, another possibility would be:
Set up your ext3 drives in server number 2 and set that up for iSCSI.
Purchase a large, inexpensive drive of sufficient capacity to hold the data on the ext3 drives, place it in server 3 and copy the data via iSCSI from the ext3 drives in server 2.
Set up the Ext3 drives in your ESXi server and let it reformat them into a datastore.
Copy the data via iSCSI back from server 3 to the new datastore.
Return the other 2 servers to their normal duties.