There is no law prohibiting them from allowing you to do this. There is also no law that states you are owed the privilege to do so unless the employer has a clearly defined policy in place that allows non-FMLA employees to make up time; if that is the case they would owe you the same luxury.
Making up FMLA time is something of a grey area legally, and quite frankly except in the circumstance described by the above poster, the employer is on firmer legal ground denying it than allowing it.
There is no FMLA law that says it must be allowed & there is no FMLA law that says the employer cannot allow it (though all employees must be treated equally). Many times it is in the employer's best interest not to allow it.