How to add a set path only for that batch file executing?

Just like any other environment variable, with SET:

SET PATH=%PATH%;c:\whatever\else

If you want to have a little safety check built in first, check to see if the new path exists first:

IF EXIST c:\whatever\else SET PATH=%PATH%;c:\whatever\else

If you want that to be local to that batch file, use setlocal:

setlocal
set PATH=...
set OTHERTHING=...

@REM Rest of your script

Read the docs carefully for setlocal/endlocal , and have a look at the other references on that site – Functions is pretty interesting too and the syntax is tricky.

The Syntax page should get you started with the basics.