Yes you can mix Roundup with diesel fuel.
Mixing Roundup weed killer with diesel fuel makes the roundup weed killer even more effective at killing weeds.
Even just diesel fuel is a good weed killer and will kill off weeds pretty fast but the addition of Roundup along with the weed killer is a good way to kill the weeds faster.
You can mix a gallon of diesel fuel per 100 gallons of roundup weed killer spray.
The weeds will die off much faster when you spray then with Roundup and diesel fuel together.
You can also use Vinegar, Salt and dish soap as a natural weed killer that is less expensive and better for the environment as well.
But diesel fuel is a good additive to Roundup weed killer.