Add Number to First Number on Each Line of File then Write to Another File - Python
Constants uppercase

A very widespread convention is that constants should be uppercase, so please conform to the standard for readability.

Unused variables

hdrln = 1
llen = 0


The first one is never used, the second one is never used with the original value, so I suggest removing that declaration.

with and iterable interface for files

with is a commonly used to keyword to simplify working with files to take care of automatic closing, also files are iterable so you can just do: for line in file that should simplify your code.

Isolate logic in functions

tfile.write ('{:{sfw}.{sprec}f}{}'.format(float(line[:fw])+add, line[-llen:], sfw=str(fw), sprec='2'))


That should be in its function along with documentation because it is the main part of the program, the logic should be highlighted and explained not hidden inside the boilerplate.