iTunes only lets you sync your iPhone with a single iTunes Music Library. This process allows you to sync music from a single iTunes Library to your iPhone, potentially wiping your iPhone's library in the process:
With iMazing, you can safely and easily transfer music from any computer's iTunes Library to your iPhone, iPad, or iPod, and preserve all metadata (e.g. artworks, ratings, play counts,…)
Anyone who wants to safely and simply copy music from any PC or Mac iTunes Library to an iPhone, iPad or iPod.
Download and install iMazing.
To transfer music from iTunes to an iPhone, iPad, or iPod Touch:
Launch iMazing and connect your device.
In the left sidebar, select Music.
In the bottom toolbar, select Import from iTunes. This will open the Music Import Wizard.
In the wizard, select the tracks that you would like to copy.
Tick the 'Music' checkbox under 'Library' if you would like to import your entire Music Library.
Click Copy to Device. This will start the import. When your files are transferred, they will appear with an iTunes icon.
Some music in your iTunes Library is considered non-transferable, and cannot be copied to your device. There are two reasons why a track is considered non-transferable:
Check the Only show transferable media checkbox to only display tracks which iMazing can extract from your device.
Non transferable media appear in light grey.