Use rules to append and and prefix character to that known part of the password. $x - will add character x to the end of the known password
$x $y - will add character xy to the end of the known password
^x - will add character x to the end of the known password
^x ^y - will add character xy to the beginning of the known password
In addition your "known part of the password part" should be on a text file. (i.e. Dictionary file) configured in john.conf/john.ini, same with the rule to be used.