Unless the voucher specifically says that it can be redeemed only at an AA ticket counter, then a travel agency can redeem it. Above posts are correct that agents do not get a commission on the amount of the voucher and are therefore likely to charge a fee to do it for you.

Also, if the travel agency is eligible to redeem the voucher, it can also issue another voucher if you don't use up the full value. However, that, too is a time-consuming process for which the agency gets nothing from the airline and may assess an additional fee or even refuse to do it.