Solution
We call up a JavaScript function that reads the Excel file via Excel automation and then inserts the values into the grid cells via SAP GUI Scripting.

Depending on the SAP transaction it may be necessary to press "Enter"  in our script from time to time after having filled all grid rows that are available on the screen. The "Enter" then provides further empty rows at the end of the grid so that we can continue with the next set of  values.
This "packeting" considerably increases the complexity of our solution, but it is necessary unless we limit ourselves to a relatively small number of rows in the Excel file.

Example
We want to upload purchase requisition data in transaction ME51N:

Our Excel file contains the following sample data:

The following video shows the uploading process;  see the full screen mode to watch it in detail:

Video

Show video in full screen

.zip file with InputScript, JavaScript and sample Excel file

GuiXTScript

if X[GRIDCONTROL]
 
 // enlarge grid control so that the user sees all columns
  pos X[GRIDCONTROL] X[GRIDCONTROL] width=220

 // add an upload button right hand side of the grid
 Pushbutton X[GRIDCONTROL]+(0,222) "@J2@Excel upload" _
     process="me51n_excel_upload.txt" size=(2,16)

  // handle further data packets
  if V[continue_excel_upload]
      clear V[continue_excel_upload]
    title "Please wait.... uploading Excel values"

    CallJSAsync excel_me51n_continue_upload
  endif

endif

InputScript "me51n_excel_upload.txt"

// --------------------------------------------------
// Upload purchase requisition items from Excel
// --------------------------------------------------

// select a file
SelectFile name="xfile" _
  title="Select the Excel file to upload" _
  filter="*.xlsx" _
  directory="C:\Puchase Requisitions Excel"

// any file selected? else no action
if not Q[ok]
  return "S: No file selected" -statusline
endif

// upload Excel file via JavaScript
CallJSAsync excel_me51n_upload "&V[xfile]"

Return

// --------------------------------------------------
//  Continue uploading purchase requisition items from Excel
// --------------------------------------------------

Set V[continue_excel_upload] "X"

Enter

JavaScript function

Components
InputAssistant + Controls