As detailed, create a file share on Server1, ensure that the nessesary permissions are set on the file share, then use a UNC path to access the server like the below in your flat file connection manager.

\\Server1\ShareName\FlatFile.txt

If you have multple flat files to import use a for each loop, passing in the file name as a variable and using an expression to the flat file connection manager so that the path changes dynamically with each file read from the share.