The only workaround I have for me is to add a scrip with a period and followed by new allergies, eg .New allergy amoxicillin. Then I add it the next office visit. I also use this method to mark the name of the patients pharmacy - works good in small town, eg. .Smith Drug, etc.
I hope as Jon develops the e-prescribing that will get better as one of the requirements is allergy and formulary checking - makes you enter allergies as one that is preentered, eg PCN, SULFA, etc.