Generally, a hearing officer may admit any testimony and evidence that he or she feels is pertinent to the case. Most will allow testimony like this without corroborating documents. An appeal to this denial would have to allege that the hearing officer abused his or her discretion in allowing this testimony or in relying on it for the denial, and this is difficult to win. You need to get an attorney to advise you about the process and standards of appeal in MO.
At the hearing when the evidence was introduced, did you deny it? Did you ask how the employer "knew" you had done that? If the employer relied on someone else's knowledge that would be hearsay. Check with a MO lawyer on the standards for an unemployment appeal. You may be able to argue that it was error to base the decision on unsupported evidence if the employer did not claim he had personal knowledge.
This answer does not create an attorney-client relationship. It is advisable to consult with an attorney with full disclosure of relevant facts for a comprehensive leagl opinion.