Now,Lets Hit it
1. Here you have to register an array of Tetxbox from which you want to take the value as well Textbox in grid where you want to put value.
put this code in Page_Load Event or rowdatabound event.....
2. Now comes the design part where you have to just assign a Tetxboxes from above code one must easily understand that which textbox is which ( ok ok dont confuse yourself...........)
And above modalpopup is into the gridview's column (templete column)so for each row 1 modalpopup will generate and you are just assigning the value of 1 textbox to other......
hope this helps some one.
Note: Becareful with the code,cause as it is generating the modalpopup for each row in my case i have data also in my modalpopup, so i am forced to do like that ,