See 'Add a row to a table in a protected form' at:
http://www.gmayor.com/word_vba_examples.htm
For the summing problem, perhaps the simplest solution is to split the table (which will also make it easier to add the new rows), bookmark it, then use a formula field coded like:
{=SUM(MyTable C:C)}
where 'MyTable' is the bookmark name and 'C' is the column you want to sum.