Yes, it is relatively easy to add manually since it is not tied to the other modules but not via the XAMPP installer or XAMPP method.
1. Download the server: https://filezilla-project.org/download.php?type=server
2. Create a folder in the XAMPP root ("\xampp\FileZillaFTP")
3. Start installer, after the 'I Agree' > check only "FileZilla Server" and "Administration Interface" > next
4. Set the destination folder to the folder you created in step 2 (if you installed to C:\xampp, it would be C:\xampp\FileZillaFTP) > next
5. Select the "Do not install as a service" option and uncheck "start after setup completes" > next
6. Select "Start Manually" and uncheck "Start interface after setup completes" > install
7. Open the Control Panel config (\xampp\xampp_control3.ini)
8. Under [EnableModules] set FileZilla=1
9. Under [BinaryNames] change "FileZilla=filezillaserver.exe" to "FileZilla=filezilla server.exe"
If the filezilla installer added any startup entries, you can remove those as well.