Can an employer commit contractually to a multi-year employment agreement in CO, considering it's an At Will state?

Yes.   At will simply means that an employee can be fired for any reason (other than for reasons prohibited by statute, such as race, religion, etc.) at any time UNLESS there is a contract which limits the employer's right to fire him/her.  A contract which, for example, provides that the employee will be employed for a term of five years and can only be terminated for cause as defined in the contract, would limit the employer's right to fire the employee.