The Pro 6 Plus is equipped with Bluetooth version 4.1 BLE, which lets you connect to your computer wirelessly. Just pair phone and computer together and drag and drop any music file from your computer to your Meizu Pro 6 Plus.
If the above-mentioned tips fail or if you prefer not to buy new accessories, you can always use one of the numerous free '3G Uploader' services available online. A search on Katapulco should retrieve plenty of them. 3G uploaders are simple: Transfer your own ringtones on their web site and they will provide a URL. Navigating to that URL with your Meizu Pro 6 Plus's browser will download the free ringtone.
More on free ringtones
Here are links to some of the 100% free ringtones (Meizu Pro 6 Plus-compatible) offered here at MartinsRingtones.com:
Free 'Unclassified' Ringtones Free Tropical Ringtones Free Ringtones-a-gogo Free Techno Ringtones Listen To All Our Free Polyphonic Ringtones