Hi Stéphan,
for sure you can do this approach and reach your target system state in two steps. Possibly this is not that complex as the CUUC approach starting on release 4.6c.
Is your system a single codepage system? In this case you do not have to be concerned about the supportability of this procedure.
Single Codepage Systems are supported up to basis release 740. If your system differs (MDMP, Blended Codepage) then you have to convert to Unicode first or reconfigure your system to a Single Codepage System (which results in possible data loss).
So you can do a normal system upgrade to EHP7 (Non-Unicode) and start a Single Codepage Conversion afterwards according to the guides attached to SAP note #1051576.
Best regards,
Andreas