There is no GUI option to set this. Shouldn't be too difficult to add.
If you'd like to review the code and submit a Pull Request we will review it.
Pending implementation of such a feature, you can edit
/etc/pihole/setupVars.conf and add as many custom upstream DNS servers as you like.
This is a feature of the new web interface, which is not yet released.
Would you be kind enough to give an example? Is it implemented yet?
1/2/1/2 or 1/1/2/2 or 1/2/2/2