Yes it is possible .....do not use a mailto button ...use a submit button ....you can set the submit url to a mailto url where you can set all parts of the mail message and you can set what is to be submitted to whatever you want. Just do a google search on the net for mailto and you will see how to set the url to what you want.
Now to set the command dynamically you can use this command:
On the exit event of the dropdown list you woudl add the code I mentioned earlier. When the user leaves that field the value of the DD would be tested, the email address would be updated and then you can programmatically hit the submit or have the user do it on his/her own.