I haven't worked with uMarketPlace but one approach that comes to mind would be to identify how uMarketPlace assigns products to vendors. If it's by category or by some product attribute then all you'd need to do is assign the values to the product during import. If it's another table containing the relationships then you'll need to write a routine to add the data there. First you'd need to create an extension for your product importer (there are several examples out there on how to do this to get you started) and just add in the necessary column(s).
If this is your import file then you could parse it into a normal import routine and programmatically assign the values that link it to the vendor. The vendor My Account section would just need an uploader and parser which should be easy enough to create.
sku | product_name | price | in_stock
1234| test product | 1.00 | Yes
Parse the csv file into an array then iterate through it to create the products.
// Add all of the minimum required fields to make your product visible and enabled. etc etc
// Now add the field(s) that link the product to the Vendor. If it links them through another table then you'll have to add the relationship there.
You'll need to add sanitizing and validation routines on the upload and error trapping to let the vendor know when there's an error.
It's not a small task but not too difficult to pull off if you figure out how the products are assigned to the vendors. The rest of the code to create your extension, add an uploader, parse the file, sanitize your data and to create your products is out there so you shouldn't have to reinvent the wheel to complete this.