Backward Mains Transformer
A mains transformer will work fine connected backwards -BUT-
The little oscillator driving the transformer will only be capable of a few-hundred mA, trying to use a transformer with a 10A secondary will provide such a low impedance as to completely short the output.
You need to choose logically - a small transformer as used to power old cassette recorders or tiny radios would be best - a 9v secondary capable of 100mA ? or 50mA ?
Experiment to find something that works (240v to 3v tx backwards with 9v in could give 600v out ??? [yes you are pushing the insulation beyond its design] )
Don't expect the full voltage out - you will have losses.
Don't expect more power out than you put in (100mA to a 24/1 ratio transformer will give 4mA out ???)