How can I backup/synchronize between a ZIP file and an FTP server ?
Sorry, this is not supported (in either direction). The internal design of all versions of SyncBack are such that one side - usually used as Source - is fixed as using a standard local disk or UNC path. The other side of the code can also use such a connection, but can optionally also 'plug in' various other modules (FTP engine, Compression engine and in the Pro version, raw optical burning, email servers, Cloud and location scripts).
Given that only one side of the code has a 'socket' for such plugins, you cannot plug in an external module on both sides. Thus, you cannot use Compression on one side and FTP on the other (in either direction). Nor can you synchronize between 2 Zips (one each side) or two FTP servers.
In SyncBackPro, you cannot back up from FTP to CD/DVD or to an email server - and so on. Basically, one side must be standard disk or network access, without Compression.
If you are looking for a method of using Compression on an FTP server (both modes on the same side of the profile), see here:
Always test first using files you can afford to lose!