SAP EXAM WITHOUT TRAINING OR AN EMPLOYER

Hi all.I am a student in usa doing masters.When in india I took ABAP training for 6 months.I am planning to give ABAP exam this august(In USA).can I do that without undergoing any training in any of the SAP institutes in USA,if not,do I need an employer's letter stating tht I can give the exam.Pls let me know as soon as possible.

Thanks,

Regards,

Sandhya.