For each field that gets transferred, you can add custom code to manipulate the field. You can use any valid python command that can be run en the eval function. The context is mapaction in transferrunner. The field value before the mapping action is sval. The sval is by default a string from the input source field.

Example of how to take a floating point string in the format 123.45 and make it into an int value of 123.

int(sval.split('.')[0])