I am trying to sync the customer file (ARCUST) in our accounting package (SBT Pro Series) with a public folder on our Exchange server. The Foxpro table uses a data-entry character field as its unique identifier. The accounting package ensures that a duplicate key is not used when entering a new cusotmer record.

Foxpro internally manages records with an field called RECNO which is not selectable in the primary key drop-down list in Genius Connect.

How do I create a primary key for this table to allow synchronization? I am not able to modify the accounting package to add an autoincrement field. Any help will be greatly appreciated.

What would you do with RECNO, It is not a saved value. Instead it is a calculated value (calculation is done by physical position of a record which may change).
Instead use a column or a combination of columns that uniquely identifies a row within a table.