howto add a sub-directory to the searchpath / namespace ?

[stef mientki]

hello,

my program has become a bit large,
and now I want to split the files over several subdirectories.
So in the example shown below, I just moved the files f1.py and f2.py to 
a deeper subdirectory.

basedirectory\
    mainfile.py
    file1.py
    file2.py
    subdir1\
       __init__.py
       f1.py
       f2.py

Now I don't want (even can't) change my program,
to change imports from
    from f1 import something
into
    from subdir1.f1 import something
simply because f1.py and f2.py are python files dropped by users
and I do not know on forehand what will be dropped.

I looked into the description of __init__.py,
in the hope I could make f1.py and f2.py available as if they were in 
the basedirectory,
but i couldn't find a way.

Is there a way to make f1.py and f2.py available as if they were located 
in the base directory,
without knowing their names (so in general all py-files in the subdir1)  ??

thanks,
Stef Mientki