Either render to audio track and then merge the result to mono,
or edit the synth actually playing the drums to be panned mono (more difficult if you don't understand synths or are using one that makes this complicated or impossible)
Or use a 'Y' cable on the output of the drum synth befoe the mixer (if separate from the keycoard)

Lotsa options, but without knowing more details about setup we can't really help you choose the best option.

I do not work here, but the benefits are still awesome
Make your sound your own!