You can certainly use one table to list customer sales and a related table to list all branches within the company. It sounds like you have two tables with identical sales data except for the branch where the sales transaction took place.
If so, then yes, you'd add field to identify the sales branch to one of the two tables, use something like Replace field Contents to add a BranchID to all the existing records, then import all the data from the other table and use Replace Field contents again to update the found set of newly imported records with the other BranchID.
You can also use the calculation option to strip the branch name out of the customer name field once you have successfully merged your records.
Note that replace field contents can modify thousands of records in one go and cannot be undone. It's a good idea to research this option in FileMaker help and save a back up copy before trying this out for the first time.
Also, once you have merged the data, be prepared for the need to modify other parts of your solution in order to successfully use the single merged table in place of two separate tables.