One way you can do this is to use a derived DataGrid and override the ProcessCmdKey. In your override, catch the Enter key and use SendKeys to change it to a Tab key.
protected override bool ProcessCmdKey(ref System.Windows.Forms.Message msg, System.Windows.Forms.Keys keyData)
{
if(msg.WParam.ToInt32() == (int) Keys.Enter)
{
SendKeys.Send("{Tab}");
return true;
}
return base.ProcessCmdKey(ref msg, keyData);
}