Here's an idea with no guarantees, and which doesn't solve the Playlist issue.
I'm assuming that you have the ALAC files already ripped and just want to import them now, plus copy the metadata mp3 -> ALAC, and include both in your library. That's how I read your post anyway.
So...
1. Select all the Albums of MP3's that you want to process, and convert them to ALAC. In Convert Options, you should set a target location that is under one directory with zero levels of folder structures (all files in one directory) so these will be easy to find and replace later. Leave Original File, Add to Destination to Library. Convert.
2. So now you have both MP3's and (crappy) ALAC files of each track in the MC Library, with the same metadata except for Playlists. I haven't checked if any other metadata is missing, so you would want to test and do a comparison. Using recently imported, select all, run "Update Library (from files)" just to make sure that all tags are in the library.
3. Turn off auto import so that MC won't update for external changes in the next step.
4. Close MC completely.
5. Copy all you new good quality ALAC files over the just converted crappy files. Okay, this process does assume that the file names are identical, so you will need to make sure they are as part of your ALAC ripping process.
6. Start MC and select all the new good quality ALAC files. They should be easy to select in the Recently Imported playlist, or using the File View to drill down to the directory.
7. Run "Update Files (from library). Now you have good quality ALACs with all the metadata that the MP3s had, theoretically. NOTE: Check files based metadata is correct, for bitrate and similar. You don't want the crappy ALAC files data written to the new good quality ALAC files. But I don't think MC will overwrite that data. After this step you may need to run "Update Library (from files)" again to get the correct bitrate and such into the library. Not sure.
8. Do magic to get you Playlists assigned to the files.
9. Use RM&CF function, with metadata, to move new ALAC files to their correct locations, be that next to the old MP3s, or some better location.
10. Turn auto import back on if you had it on originally.
Done.
That is a lot of steps, and I haven't tested if it would work completely as expected. Do a small test. If you only had a few CDs to process it wouldn't be worth it. But if you have a lot, you can process them all at once using this method.
Step 8 is the hard one. Without lots of expression magic I can't think of an easy way to get the new files assigned to the appropriate playlists. But here are some ideas.
a. If you have moved the new ALAC files into the same directory as the old MP3 files, select an MP3 playlist. Export it. Edit it externally with a suitable editor. Globally change "mp3" to "m4a", assuming that is the extension you used. (The MC Convert process creates an m4a file with ALAC compression.) Import the Playlist back into MC. i.e. What Jim said.
CHECK: If you want both the MP3s and M4A files in the one playlist, you may need to have both the MP3s and M4A files listed in the playlist for import, so that the playlist in MC doesn't get overwritten with just the M4A files. If so, just copy the MP3 section into the file again after the global edit. I'm not sure if you want both in the one playlist though (I wouldn't) so maybe just do the global edit above and change the playlist name to something new, so you get a new playlist with the same contents as the old, but with ALAC files instead.
b. Set up a two pane view, side by side, and display a playlist on one side, and the recently imported files on the other. Then select each files you want in an ALAC playlist, right-click Send To new playlist.
c. Use the find duplicates functionality to list all the now duplicated MP3 and M4A tracks, then check each MP3 to see which playlists it is added to, and add the corresponding M4A file to the same or a new playlist.
Once all this is done you could Stack the MP3 and M4A files if you wished.
DISCLAIMER: These are just my thoughts. If it breaks everything, it isn't my fault!
BACKUP YOUR LIBRARY AT EACH STAGE.
EDIT: I see Jmone has beaten me and said something similar, in many fewer words! But I was assuming, maybe incorrectly, that you wanted to keep the MP3 files and manage them in MC, as least initially.