Amphetamines are readily detectable by chromatography in hair for three to four months following termination of use. Positive results involving periods longer than this will depend on factors such as hair length, hair type and hair growth rate etc. There are documented cases in biochemical pathology journals of amphetamines being detectable for around nine years. The chromatographic equipment and facilities required to give the result is not required to be state-of-the-art and amphetamines are readily detectable using relatively vintage equipment via hair.
Having worked at a number of USA universities, I've experienced at first hand the use of Adderall on campuses and witnessed the ease of detection. A capable technician should routinely suspect illicit substance use from the texture of your hair, so yes, it is feasible that the hair sample could be taken from elsewhere on your body.
I appreciate you may not want to hear this, but I think it unlikely you will pass the test based on the information you've provided.