Purpose
You want to let the user start a GuiXT InputScript by clicking a button in Excel

Solution
Assign a macro to the button that starts "guixt.exe" with the option "input=..."

We assume that a SAP GUI session is already open for the system in which you want to start the InputScript.

Example
VB.NET 

Sub upload_items()

  Call Shell("C:\Program Files (x86)\SAP\Frontend\sapgui\guixt.exe" & _
                     "input=OK:process=C:\guixt\excel_upload_items.txt", vbHide)
End Sub


Starting a series of InputScripts

If you want to start a series of InputScripts with the button click, for example one InputScript call for each Excel row, you need to synchronize the InputScript execution in your Excel macro. Otherwise a running InputScript could be interrupted by the next one and deadlock situtations may occur.
A possible approach for synchonization is to use an empty status file that is written when the InputScript has terminated. In the Excel macro you wait until you detect the status file:
 
VB.NET 
' Wait for Status file to confirm that the InputScript has finished
 Do
   Application.Wait Now + TimeSerial(0, 0, 0.01)
   FindIt = Dir("C:\Guixt\excel_status_file.txt")
Loop While FindIt = ""
' Now we can start the next InputScript
 

At the end of the InputScript, set a GuiXT status variable

 Set V[excel_status_file]   "C:\Guixt\excel_status_file.txt"
In the GuiXT script that follows your InputScript you query the status variable, create the file and clear the status variable:
GuiXT 
if V[excel_status_file]
  OpenFile "&V[excel_status_file]"   -output
  CloseFile "&V[excel_status_file]"  
  Clear V[excel_status_file]
endif
This way your are sure that the InputScript has ended when you write the status file.

Components
InputAssistant