Hai,
You can achieve it but it has its own disadvantages.
Whn u run the TMG thru sm30 , place the cursor on one of the field in the scree in which u need to create a button and press F1 that gives u the technical info of the field.In that u can find the first block screen data which include the screen number and teh Program name.
Double click on the screen number that takes u to the screen of that program.Now u can take the Layout of that screen ,create ur button require in the edit mode and activate it.Thus u get ur button in the TMG.
Disadvantage :Lateer if u regenerate the TMG the coressponding screen in which u placed the button will be regenerated and hence ur button also will get deleted.
Regards
Neeraj
