To enter previous flight information without entering each flight, you need to group fights according to aircraft classification then total each group and enter it as one "long" flight.  The more detailed your grouping is, the more detail you will retain in your Tally Sheet totals (and the more work you have to do).  Let''s take a simple example.  Say the last page of your first logbook shows the following totals.

Landings:     594
Day:      235.4
Dual:      81.5
Inst App:    66
Night:       22.7
PIC:       78.0
AIR-SEL:    255.7
Hood:      43.3
X-Country:  136.2
Actual:     3.7
Total Time: 255.7

Looking at the totals, you can tell right off that all time was spent in Airplane-SEL aircraft since this total is the same as the flight time total.  This means you can be really lazy and get away with one multiple-flight record for a single engine land airplane (Category=AIR and Class=SEL).  Even with one record, you still have to flip through the book to get information on how to divide the totals in the breakdown table (e.g., how many landings are at night, etc.).

This grouping gives more detail than the original logbook, but far from the detail possible with AeroLog.  There is no breakdown by make/model (type) since all flights in the various types of aircraft are lumped together.  To include this level of detail, you would have to scan through your logbook and calculate a separate set of totals for each type of aircraft you have flown.  The number of different types would determine how many multiple-flight records you would need to create.

The finer you make your groupings, the more work is
involved.  Essentially you are doing by hand, what AeroLog does for you automatically when you enter each flight.  At some point it becomes so involved that it will take less time to enter all the flights from the beginning. The bottom line is detail.  The more you want, the more time and effort you have to put into the process.  If you are short on time (and who isn''t these days) you can establish a baseline using a  few multiple-flight records to start.  As time becomes available, you can gradually replace each multi-flight record with the individual flight records it represents.