The way you would normally go about this is to utilize a query parameter, say "tblid" to distinguish between add and update mode.
Using the form you already have if the query string parameter "tblid" is empty then you are in add mode. Thus when user clicks on the Submit action button you add a new record to the your table, assuming you are already using the Execute SQL workflow task. And if the querystring paramter "tblid" is non-empty you load the data from your table using a stored procedure and bind to the fields. Whether the "tblid" is empty or not, you will still need to pass the querystring param to the stored procedure in the form of token. Assuming the load stored procedure is called "myLoadSP", you'd call this as myLoadSP [QUERY:tblid]
As for the datagrid that displays all the records in your table, you simply add new column that provides a link to the same form with the querystring "tblid" embedded.
Hope that helps