Just increase your block size for the file to be a bit more than size of the file. This will ensure that the file goes to single mapper. This has to be done when putting the file in HDFS.
As you indicated since these files are generated by another MR:
You can create your InputFormat and override the getSplits() method.
The getSplits() returns an array of InputSplits. Return a single split instead of breaking it into multiple parts.