If you are using MultiBit HD, it is not possible to create the wallet files easily by hand. You can't import keys into MultiBit HD. If you are using MultiBit Classic, then make a text file with the following format for each key, one on each line:
The <timestamp> is the ISO 8601 timestamp including the century. It should look something like 2016-10-06T21:03:15Z. That timestamp is in UTC and is used as the starting point to start scanning for your transactions.
Save that file as a .key file and then you can import it to MultiBit Classic.