Can you use a 60Hz transformer in a 400Hz low power system?

There's no problem except that the transformer will be more massive than necessary. It will work at the same voltage and current, but there is less iron loss, so the voltage rating can be increased by a maximum of 15-20% and the current rating can be increased by 15-20% also.

But if this is in an aircraft or other safety-critical application, do not overrate the transformer.