FileZilla keeps its user data in C:\Program Files (x86)\FileZilla Server\FileZilla Server.xml
You can't import the FileZilla XML directly into CompleteFTP (yet!), but if you convert it to a comma- or tab-separated text file then this can be imported. The format of imported or pasted text must be as follows:
1. No header line
2. One user per line
3. Fields are separated by commas or tabs
4. First field contains username (mandatory)
5. Second field contains the plain-text password or MD5/SHA1 password hash (optional)
6. Third field contains Windows path of home-folder (optional)
7. Blank lines are ignored
This is available in the 'Add multiple non-Windows users' feature.