clean install of win8 and outlook 2010. re-use a 4gb imap gmail pst file (among others)?

Hi,

Before clean-installing Windows 8, I dutifully created a backup of my files including all my pst files in the appdata folder for outlook. these include a separate pst file for each of my emails (yahoo, gmail, my university email, my isp email). all i prefer to keep online - that is, i do not actively "archive" locally. i do not create a unified inbox, i just like them separate.

Outlook 2010, which I was using on Windows 7 (home premium) and have re-installed on Windows 8 (pro), had created individual pst files for each of my IMAP emails. This is fine. Although I specified to keep copies of emails on the servers (but delete there if i delete locally), Outlook's pst file for my gmail account is over 4gb. So, it seems to me that Outlook is actively keeping any emails it might be viewing/sending in this pst file and i assume this is what allows me to view some of my emails offline even though i don't 'archive'.

My question is: can i reuse this massive file, since i would like to keep the ability of offline email viewing of past emails, so that outlook doesn't re-download this amount of data (i am on dial up with a small bandwidth allowance).

I have tried:

1) when creating a new account in outlook, i copied the backup to overwrite the newly made, 225kb pst file outlook made for gmail. what happens is when i reopen outlook, it generates another brand new 225kb (or so?) file.

2) changing data file location through control panel | mail options. same result as (1): creates new, fresh pst file.

3) importing pst file from within outlook via file | open | import. options are selected to not import duplicates. what i think is happening is outlook is trying to update the _server_, not the local pst file. I have retried importing but there seems to be no option to save the emails into the pst file itself, only the server.

4) I considered changing the registry to force default files to go to my documents folder where i had copied the pst from my previous installation, but I suspect the outlook/pst behaviour will be the same regardless of where the files are actually asked to be stored.

why would outlook create such a large file if it cannot be used outside of a narrow definition of account management (its own)? I suspect there MUST be a way to use the data outlook has already gathered, otherwise why bother gathering it?

the files are not corrupt, as i can open the data files in outlook and view the emails there. some may ask: isn't this what you want? well, no: i want to transfer the emails from that pst data file to the new one outlook makes to prevent outlook from downloading 4gb of the same data all over again - i don't care to have it twice on the same computer...

anyone got any clues if this is even possible? or am i stuck re-downloading my entire gmail (and other accounts) each time i install outlook.....?

thanks.