You can use the sms content provider to read and write sms messages:
ContentValues values = new ContentValues();
values.put("address", "123456789");
values.put("body", "foo bar");
getContentResolver().insert(Uri.parse("content://sms/sent"), values);
I don't know why you would want to write a message you send to the inbox but if that is what you want just change the above uri to "content://sms/inbox"
.
Alternatively you can hand over to a messaging application by starting an activity with an intent similar to the following:
Intent sendIntent = new Intent(Intent.ACTION_SENDTO, Uri.parse("sms://"));
sendIntent.putExtra("address", "123456789");
sendIntent.putExtra("sms_body", "foo bar");
startActivity(sendIntent);
Edit: However, the sms://
content provider is not part of the SDK so I strongly recommend not using code like this in public applications for several reasons.