I would go for a temporary table in this case, if the processing of it takes not too long. First you are able to use the sequences like in the final table and all you need to do is issue an "insert into select from" statement once the user clicks ok in the second screen. This would be a simple solution to implement i'd say and all the processing is done.
If the processing part is "huge" and the user could check certain things before this big task a solution could be to split up the processing in two parts and store this first results in a temporary file and implement a screen based on that one. After the user clicks ok you can launch an async task that does the heavy lifting and cleans the file after hes done.