I assume that your Excel imports have a predefined structure. Now, importing Excel files using IMPORT is impossible: it requires that CSV conversion step.
You might consider doing the following:
1. Put samples of files that you want to import on your server. Put them on a location that you will not change!
2. Create ODBC connections to these Excel files >> so now you can read them out as databases and do whatever you like with the info
3. In your current or a new database, create a table where you can store files. See SAVEFILE for requirements
4. Replace your current import function with something that uses SAVEFILE: the purpose is that the Excel file is loaded into the database table
5. As soon as the SAVEFILE is done, use CREATEFILE to create the Excel sheet in the location defined in step 1
6. Query the data using the ODBC connection created in step 2
It's a bit of work to set it up, but this solution allows users to upload files, without using CSV conversion :)