In general, these kinds of motors can be used at voltages higher than they are rated for, which will result in higher speeds and more torque at the cost of their operating life. At some point, raising the voltage too much could cause the motors to fail almost immediately. While running your motors at 7.5V will lower the lifetime when compared to 6V, I would not expect any immediate problems as long as you keep the current draw to around 20% to 30% of the rated stall current.
Additionally, you might consider limiting the duty-cycle you apply with your motor drivers or controllers to about 80% to avoid powering your motors with more than 6V.
As for mounting your wheels, we do not have any 4mm-to-3mm shaft adapters. So, if your wheels do not provide a way to connect to a mounting hub, then we do not have any specific suggestions.
However, if you decide to look for new wheels, you might check out our multi-hub wheels which include hubs for 4mm D-shafts.