What you propose would break standard and expected keyboard behavior. (I don't want that myself - I like the enter key to insert a new line - I do this often in text messages)

Conversations is a closed Nokia application so I suspect at this time you cannot remap the "Send" button to the enter key.

Although I can understand the small convenience of having the send button mapped to the keyboard, is it really that difficult just to tap the send widget 2 centimeters above "enter"?

If it were possible, rather than change default behavior, it would be much better to have two options:

1. Ctrl + Enter = Send
2. A configuration option somewhere that is disabled by default which allows Enter = Send.