No you can't start XAMPP while the normal web server apache and the normal database mysql are running. They are using the same ports 80 and 3306. You will find the configuration files /etc/apache2/httpd.conf and /etc/my.cnf on your mac server.
To start XAMPP at the same time you must find new ports for XAMPP (8088, 8089). Edit the XAMPP configuration files /Applications/XAMPP/xamppfiles/etc/httpd.conf and /Applications/XAMPP/xamppfiles/etc/my.cnf. In httpd.conf you will find Listen 80 (line 40) which means that apache wait for access on port 80 ... change this to 8088. In my.conf you will see the port twice ... when you change both to 8089. Restart apache and mysql in XAMPP Control.
That' all ... you will see your web server with http://localhost:8088