Any visitor in Canada whether he/she is on supervisa or regular visa can apply for visitor visa for USA. It's up to US embassy if they will give visa to the person depending upon the person's personal circumstances.
usually when people already have visa for countries like Canada, UK,Australia, USA etc, they can get visa for other countries easily and there are better chances of getting a visa.
At the end, the consulate officer will determine whether the person will get a visa or not.
Pretty much common-sense.
Most countries require you to obtain it in country of birth/nationality.
On Visit Visas most countries do not entertain and likely refuse apps telling applicant to apply in home country.
Study/Work permits are exception ofcourse.