If you didn't sign an agreement to be drug tested before being employed, can an employer force you to sign such an agreement afterwards?

Yes, your employer may require this--the employer sets the terms and conditions of work, may change them at will, and may require employees to consent to random drug tests or lose their jobs (or face other actions--e.g. suspension, less desirable shifts, demotions, etc.).

There are exceptions:

1) If you have an employment contract, the employer can't taken action against you if that would be barred by the contract's terms.

2) If the employer is singling out a racial group, or a gender, or a religion, or employees over 40, for "random" drug testing, that may be illegal discrimination.