[Joe Forster/STA]

I think you're looking for this:

for /r %%f in (*.bk2) do arc.exe a -i2 -r -mbpk %%~dpnf.arc %%f

The variable called "%f" (if used in a .bat file, "%" must be doubled) is assigned the paths of all files matching "*.bk2" in all subdirectories, in any depth, of the current directory. After each assignment, the command after "do" is executed. The "%~dpnf" syntax splits the path in "%f" into components, keeps the "d"rive, "p"ath and "n"ame components only, and drop the e"x"tension - because you add another extension.

Run "for /?" in the command line for the complete description of the "for" command, including everything I wrote above.