You don't indicate exactly how you plan to 'merge' the two files. But you can use a global field and type data into it. This data will be the same for ALL records. Globals are specific to the User and are only session-specific so, if you close the file (when being served), you will lose the global value.
If you do need your value stored in a local field, if you have a calculation that refers to it and it needs to be a stored, indexed field for example, you can use Replace Field Contents.
This is a powerful tool, so look it up in help and experiment with a copy of your file until you are confident you've got it to work for you. I would also avoid using this tool on the file while it is being shared over a network unless I was absolutely sure that no other users might be editing one of the records as that will mess up your use of this tool by keeping one or more records from being updated.
Thanks a lot - it worked. By merging I meant I will then export that whole file into another existing file.
You may also knoe the answer to this:
Can I do a search for all records that contain 'x' in a specified field and replace 'x' with 'y' in the same field in that found set?
Same problem, same trick really. You may have to specify a calculation depending on what is in the field where you change 'x' to 'y' in your Replace Field Contents operation.