Meanwhile I created a small new partition on the system to be converted, copied all files from /boot to that new partition (while preserving access rights), corrected fstab accordingly (meaning: an own /boot-partition definitely exists by now), called mkinitrd (as I still don't know, what vmware converter really checks, just in case it checks the init ram filesystem).

Result: Linux still boots without any complaints and vmware converter still refuses to continue conversion with the message "there is no /boot directory mounted  on the source system".