drop down boxes

I assume by drop down box you mean data validation.

To do this select the cells you want the validation to extend to as well as the cell you originally created with the drop down box. Choose
DATA > Validation. You will get a message that says something to the effect that some of the cells you selected have data validation applied. Do you want to extend that validation to all the cells you selected. click yes and you're done.

Well you're almost done. Clicking yes brings up the Data Validation dialog box. click yes here and you're done.