As Luca has stated, it's the source you need to modify. Take a look at the files in your source directory. For example:
The file named check_fping.c has this line in it:
die (STATE_CRITICAL, _("FPING CRITICAL - %s is unreachable\n"),
Change it say:
die (STATE_CRITICAL, _("FPING is about to explode - %s is unreachable\n"),
or whatever you wish it to say.
Then recompile the plugin and install the new one.