The sparse virtual disk format is not supported by ESXi. What you can do - other than using the converter - is to convert the virtual disk to an ESXi supported format using vmware-vdiskmanager (see e.g. http://kb.vmware.com/kb/1023856 -> Attachments), upload the 2 resulting vmdk files to the ESXi host's datastore and create a new VM using this existing virtual disk.
André