Importing a SQL Dump File through XAMPP's Command Line
Depending on the tool and parameters used to create the .sql dump file of multiple databases, the file will normally have 

CREATE DATABASE DBn...;


and

USE DBn;


statements that will allow your import to proceed without hiccups. For example, both the mysqldump command mysqldump command and phpMyAdmin's Export function for multiple database insert them. 

Assuming you have exported with a sensible tool like those above, then you can import the database with a command line like this:

mysql -u username -p &lt; dumpfile.sql


Your mysql username account needs to have appropriate privileges to, for example, create databases.

You can even run this command in a more familiar way by naming the startingDB database to use before running the commands in dumpfile.sql:

mysql -u username -p startingDB &lt; dumpfile.sql


As the dumpfile will have a CREATE DATABASE and USE before any CREATE TABLE or INSERT statements, everything will work fine.