Sure you can create a nice up-load page.
In fact, the only confusing part here is you needing that Excel sheet to up-load data? Why is that required?
The customer would (presumably) log onto the portal, select the project or quote or whatever it is they are supposed to work on.
After they select the project, you can have a up-load file area, say like this:
The above allows use of select (to select multiple files), or the user can drag + drop files into that hot zone area.
They can type in comments etc. for these files being up-loaded. However, since the customer selected the project to up-load files, then all of the information, and EVEN the folders for that project can be created on your system, and then upon up-load, then those files are now already saved into the correct folders for that given project.
When they up-load, then we have this:
Now, I could all all kinds of things to the "row" of information for that up-loaded file. (or let the user edit the information in above, and hit save).
So, really, a asp.net web page is a "blank canvas" for you to create just about anything you want.
which could then be inserted into an MS Excel sheet, which can be used to import the data and files into the .NET application i use
Why would you need above? The web portal can update and enter that information for you. The web portal can and should operate against the same database you have now, so I don't see any need for some Excel sheet?
If the customer wants to start a new project, or up-load files to a existing proejct? Then as above shows, they can up-load files, edit the comments for each file, and when they up-loaded these files, then the web portal can create the project folders, and save the files into those folders for you - all automated.
I suspect I am not grasping where the excel sheet or data comes from? However, why not have the web portal pull that data and elminate the Excel sheet import part?
and if you have some kind of approaval proesss, put that on the web protal, and when a customer approves a project or whatever? Have the web portal go into your existing database and update that information. So any of the parts now for your desktop program? Move out the parts that the customer can do. So, in place of you opening a email, launching the desktop software, finding the project, and entering/changing the project status to approved? Have the customer do that now!!
So, you are really quite much free to create just about any kind of web page, and include some file up-load ability. You can have the user enter comments and things about the file(s) up-load at up-load time, or even let them up-load, and say display the 2nd page like above, and they can then enter information about that file or project at that point in time.
Since the web portal would be connected to the same database as your current desktop software, then no need to export/import from that Excel sheet. Everything can occur in real time.
So yes, you can certainly add the above type of interface to your existing web site, and I can't think of any real limitations in regards to asp.net vs that of your mind dreaming up a web based interface as you desire.
I think a "really" great idea is to build some type of web interface in which the customer can select a project to work on, since then any up-load, notifications to sales reps or project managers can now be driven from that existing database you have now.
And with a up-loading system as part of your site, then no 3rd party up-load site or system would be required, but MUCH better is that such files would now automatic be placed in the correct project folders for each project.
So, short answer?
yes, the above screens are simple vb code and asp.net screens.