I don't know if I have what you want, I posted several times on simplemachines and elsewhere, but got no response, so if you have questions first search for my postings.
You can download here :
http://www.amevita.eu/modules/wfdownloads/viewcat.php?cid=3the files that I used to downgrade and convert.
Use this at own risk and no support from my side.
Before you even begin make a good plan and BACKUP database /files of your XOOPS installation and actually TEST if these backups work.
For the actual conversion test if the conversion and so on works well.
For that purpose install smf 1.1.2 in a subdirectory of you XOOPS site.
Copy the database / files of your XOOPS site and your new smf 1.1.2 to your local server or test server.(for the conversion you'll have to change path / database data in the smf setting files)
So you have a copy of your site.
Copy the files you downloaded to the root of your test-xoops site.
Execute downgrade.php (you don't have to convert blocks because you will not use XOOPS after conversion)
You will get several warnings, don't mind that.
Then start the conversion by executing the conversion.php script. The script will ask you some things like where you have your smf install and so on.
Then the conversion will start, when you have a big forum this will take some time, DON'T, I repeat DON'T touch the continue button, because this will mess up your database!!!
After the conversion is complete make a backup of the smf database and restore that on your site.
Then let smf check/repair/optimise your database and do a recalculation of your statistics.
Then you need to assign right to your boards and assign moderators because these are not converted.
Don't forget to make backups between this steps and at the end !!!!!
When all goes well repeat this when you do the actual conversion.
Then, the only thing you have to do is deleting the XOOPS files (you may want to keep some directories like uploads/images because your postings may point to images inside these directories)
Then move your smf install from the subdirectory to the root of your site and run repair_settings.php (can be downloaded from simple machines org) and change your theme directories to the correct data.
Because a lot of users have your forum in their favorites, you may want to make the directories modules/newbb/ and place in that an index.php with the message that your forum has moved.
Because I had only 160 users I also converted the avatars of my users (by hand and with the help of ck-rename of hervet)changed their signatures, and installed the smilies that I used on the 'old' forum.
What also happens is that search engines like google go bananas
Succes with your conversion