By converter plug do you mean an adapter plug that attaches to the power cord from the mixer so that it can be plugged into the Spanish wall power outlet?
If so, then NO.
An adapter plug is meant to alter the physical configuration of the plug so that it fits into the intended wall outlet and that the wall outlet is connected to a power supply that has the same voltage and frequency as the country where the device was originally made for i.e. in this case the USA
You need a step down voltage/frequency converter - example only
It converts the Spanish 230V AC 50Hz power supply to the 120V AC 60Hz power required by the mixer. You also need to make sure that the converter can supply the 600 Watts of power that is needed by the mixer. 1KVA = 1000W
As you can see from the example, it would be cheaper to buy a mixer in Spain to suit your requirements.
A cheaper 230V / 120V converter would also not work as these only convert the voltage and not the frequency which if it is not the same can also damage the mixer. The frequency provided will be the frequency of the mains power supply in the location where it is connected e.g. input = 230V AC 50 Hz, output = 120V AC 50 Hz, not 120V AC 60 Hz
Here is the user manual for the mixer.