1. Unless the Excel spreadsheet resides on a server, you will have to figure out how to transfer it to the mainframe. If it does reside on a server, you can use FTP (among other products) to directly transfer the Excel spreadsheet to the mainframe.
2. There is software available that can be purchased and installed on the mainframe to allow an Excel spreadsheet to be read. Whether or not that software has a COBOL API, however, I do not know -- but you need to find out.
3. If your site does not currently have such software installed, you will either need to get your site to purchase the software or develop an internal way to read the Excel spreadsheet. The last time I looked, a couple of years ago, the Microsoft Excel file specification document was well over 1000 pages so developing an internal way most likely will take 3 or 4 people a year (or two) to complete.
4. Mainframe COBOL does NOT deal with PC software directly -- the usual way to handle Excel is to convert each worksheet to a .CSV file (so if your Excel file has 7 worksheets, you'll need to create 7 .CSV files), upload each separately, and then process each as a sequential file on the mainframe -- unless a software product is installed on the mainframe to handle the data, as mentioned above.
It sounds very much like you've got a soution and now you want to warp your problem to fit the solution. This rarely works out well.