Something is funny in your question : I was looking tonight 
for HTTrack exactly for the inverse reason !
MemoWeb is not converting .asp link  into .htm link in the 
saved pages and is storing both files (xx.asp and xx.htm). 
After mirroring the site with MemoWeb, when I click in a 
saved page on an  .asp link, the file xx.asp "is ready" to 
download from my hard drive to my hard drive. I would 
greatly prefer to see the xx.htm loading in I.E.
(asp pages are  interpreted in the server and HTML is the 
only meaningful output...)

That is why, I read carefully the HTTrack FAQ where it is 
written something very interresting for me.
In the paragraph concerning parsing time, they explain 
the "assume option" which should permit me to convert .asp 
link in .htm link and of course xx.asp file in xx.htm as I 
NEED IT. I have not yet tried HTTrack.

Perhaps you should try to force  asp to be converted in asp 
with the assume command :
--assume asp=text/asp

Here is an extract from the FAQ :

Q: HTTrack is taking too much time for parsing, it is very 
slow. 
Dynamic pages. Links with names terminated by .php3, .asp 
or other type which are different from the regular .html 
or .htm will require a supplemental request, too. HTTrack 
has to "know" the type (called "MIME type") of a file 
before forming the destination filename. Files like foo.gif 
are "known" to be images, ".html" are obviously HTML pages -
 but ".php3" pages may be either dynamically generated html 
pages, images, data files...
If you KNOW that ALL ".php3" and ".asp" pages are in fact 
HTML pages on a mirror, use the assume option:

--assume php3=text/html,asp=text/html