is there some way or workaround to pick/copy Mixes that were originally created in separate 64-songs files and have them gathered into a new 64-Songs set? In other words, re-using already existing mixes, just merged in the same set, so as to easily support a new lineup?
Is there any MOXF native way (maybe playing with Pattern/Song conversion, or using also the MOXF editor, or…?) to accomplish this, or rather is it strictly needed to purchase the Melas Librarian?

There are several ways to accomplish this.

You can use the CLEAR SONG [JOB] to empty the MIXING setups.
From your ALL data (.X6A) or ALL SONG (.X6S) Files you can load any Single Song entry.
Press [FILE]
Press [F3] LOAD
Set the TYPE = Song
This will allow you to highlight a .X6A or .X6S File and “open” it to reveal a list of Song titles (we assume you give each Song Mixing a Name)
Highlight a Song Name and Direct it into your current Song Sequencer.

Setting the TYPE is the key. By selecting to load “Song” you get to pick them each-by-each. If I understand, you are using these just for the MIXING setup, so they should load in a second or less. There are 64 PATTERN MIXING setups that can also be accessed individually from either an ALL data (.X6A) File or an ALL PATTERN (.X6P) File.

Alternate Methods:
Use the Yamaha MOXF6/MOXF8 Editor Standalone/VST you can use this to capture each SONG and PATTERN MIXING setup separately.
Organizing them on your computer. This will allow you to Bulk the SONG/PATTERN MIXING setups from your computer to the MOXF. Again, this is as quick as can be.

If you have lots of Songs (setups) you can use the Editor to Bulk the setups to the MOXF. They arrive in the MOXF’s Edit Buffer, there you can immediately play it, but it does not overwrite the current setup. If you want to keep it, press [STORE].

Cubase MediaBay > Sound Browser can organize all your MOXF Voices, and Mixing setups. Even if you don’t use the Sequencing part of Cubase, the MOXF6/MOXF8 Editor VST is your gateway to using this feature to professionally organize your data. This would be useful when you have time (at home) to backup and organize all your individual Voices and Song/Pattern Mixing setups.

Extra Credit:
MOXF Adding Voice Libraries to Cubase as VST Preset

Thx a lot Bad Mister, timely and clear!