Set up a field with global storage for your drop down menu. Use the OnObjectSave Trigger or a button next to the field to perform a script that performs the find. The data in a global field can be accessed by your script when in find mode and from any layout in your file.
The basic script for this looks like this:
Go to Layout [Layout you want to search on]
Enter Find Mode ----> celar the pause check box
Set Field [YourTable::YourField ; YourTable::GlobalField]
Set Error capture[on]---> keeps dialog from interrupting script if no records are found
Perform Find 
IF [ Get ( FoundCount ) //use if statements with Get (FoundCount) to determine if 0, 1, or many records were found and have your script take the appropriate action for each ]
To see examples of many different ways set field can be used to specify search criteria go to this link: http://forums.filemaker.com/posts/ba7347f58a and check out the sample script posted near the start of this thread.
Perfect, thank you.
One other quick question. If I select Prohibit modification of Value during Data Entry will fields still update when I do the matching import?
Yes. data entry is specified as a user editing data on a layout. Scripts can still modify the value and so can data imports.