Following up on Wayne's comments, yes it is
possible but with any long history or lengthy
experience it is time consuming, tedious and
probably requires some short cuts/creativity.
Earlier this year I entered "most" of my data from
40 years of birding in many states and countries.
I am not a county or state list keeper (except
Washington) so that complication was avoided. All
listing is done on a "trip" basis so it is
necessary to identify where and when you first saw
a particular species but it is not necessary to
add every species you saw at that place at that
time - which I do not have data for, but others
might. My goal was just to get each ABA area,
country and State of Washington species from my
life list accounted for. So for example my first
Snowy Owl sighting was in Magnuson Park in January
1974 so that can be entered as a single sighting
for that location as an "incidental" trip without
also entering other birds seen then. It will then
show up on World, Country, ABA and State of
Washington life lists. I essentially had to do
this for every State and ABA bird seen over the
past years - sometimes with multiple "firsts" from
the same "trip" but it was a lot of work and was
only as good as my early records.

In the case of birds that were first seen
elsewhere in the ABA area but then again later in
Washington (again the only state that mattered to
me), there had to be two "incidental" entries if I
wanted credit for both "life lists".
Unfortunately my early Washington state records
are often incomplete, just being a tick and not a
location and time if I had already seen the bird
elsewhere in the ABA area and entered that in the
records I kept at the time, I did not want to
make up "bogus" trips, so my Ebird list for
Washington is incomplete...at leat numbers wise I
am trying to make up for some of that this
year...if only those rarities (for
Washington...but previously seen elsewhere in ABA
area) will show up again...

I expect there are other ways to approach these
matters and I will look for other responses to
learn from the experience.