After a couple of years of delay, I've upgraded to High Sierra.
Now, I have an external disk used for backup, with the following layout:
/dev/disk6 (external, physical): #: TYPE NAME SIZE IDENTIFIER 0: GUID_partition_scheme *2.0 TB disk6 1: EFI EFI 209.7 MB disk6s1 2: Apple_APFS Container disk8 80.4 GB disk6s2 3: 00000000-0000-0000-0000-000000000001 1.9 TB disk6s3
Partition #2 was an HFS+ file system that I've converted (by re-formatting) to APFS and contains a clone of my macOS partition.
Partition #3 is used as a VeraCrypt container, formatted as HFS+. So far, I was unable to reformat it to APFS because the Disk Utility tool complains that "a whole disk is needed" for the operation. Also the command line failed.
At this point, I think I've understood that the operation is not possible. Not a big deal, since this a backup disk (with rsync) and APFS would bring only a minor improvement. I'd only like to know whether I've understood it right or not.