4
While I don't have an exact answer for you, I can give you some pointers unless someone else can give you the code.
You will need to edit both the code for the block (in /modules/system/blocks/system_blocks.php, find the b_system_search_show function) and it's associated template (/modules/system/templates/blocks/system_block_search.html, but done via the database or your themes).
In the block code, you will need to run a SQL query to get the active modules similar to what is done in the xoops_module_list function found in /modules/system/admin/modulesadmin/modulesadmin.php. Then you load that into an array (say as $modules) and pass it into the $block array (say as $block['modules']).
In the HTML template, you will need to see if $block.modules exists, then loop through them into a drop down select button.
Hope that helps, some. If you get it figured out, then please post here for everyone else to help out. It's a good idea, and similar to what I was trying to do recently with a theme solution to be able to search the current module, but even enhances upon that concept.