Is it legal for Human Resources to require employees to sign revised job descriptions with more requirements than when hired?

Assuming the employer is not doing this for some nefarious reason, then yes, they can add duties to the original job description.  If the duties are significantly more complex, you could possibly ask if there is additional compensation for the more complex duties.  They would not have to provide this, however.  If you feel that the company is doing this to set you up for failure based on discrimination or retaliation, then there may be legal action you can take. You should meet with an attorney to review this in more detail.