MySQL: Merge Same Database on Several Location into Single Location
Let me first suggest an approach that may not work for you, but it seems the ideal approach for saving bandwidth and resources:

Multi-source replication is a relatively new feature, available on MariaDB 10 and MySQL 5.7 (not yet released). It is used precisely for the case you are talking about: merging data from different servers into one, typically for analytical purposes.

Here it is an overview of how it works for MariaDB 10 and for MySQL 5.7. If the tables have the same name, and you set the proper filters, you could have the table almost up-to-date in real time without having to do imports and exports every single time (which will become more and more inefficient each time). Even if you do not want that, it would be just easier to start and stop the replication each time, unless the tables are completely rewritten between synchronisations.  

As writing to the same table could become insecure (for the data integrity)- depending on the kind of DML queries that you execute at each location-, you could alternatively write to separate tables and use a VIEW to see it as a single table. This may or may not be better for performance depending on the queries executed on the central location.

If this does not work for you (for example, because you cannot use those versions of MySQL- after all, this is a relatively new features), I can suggest alternative methods.

EDIT:

As you seem to not be able to use a stable internet connection (although that wouldn't be a problem, replication is asynchronous and it will work even if it stops very frequently), I will assume you want to do the transfer via a USB drive. 

My second recommendation would be to backup the table or tables using CSV if the data is not too big, and in binary format if they are relatively big. 

For CSV format, you can use mysqldump --tab or mydumper and then import them back with mysqlimport/myloader.

If CSV format is too slow, I would recommend you to use something like Percona XtraBackup that, combined with InnoDB, allows you to export tables in binary format and then reimport them separately (it requires Percona Server or MySQL 5.6 on the importing node and innodb_file_per_table). You can first import each table separately and then merge it into a single table as partitions with EXCHANGE PARTITION. That would allow you to do the export/import at nearly disk speed.