Can an ac adapter's output be AC?

Yes an AC adaptors output can be AC. In order to change the output to AC a few components inside of the adaptor has to be removed. Pretty much all that has to be removed from the adaptor are the diodes. The gaps in the circuit then have to be bridged in order for the electricity to continue to flow to the output plug. However this can cause problems with the circuit causing the capacitors to explode. I dont recommend modifying an adaptor to output AC. Just go out and buy another one.