Yes its possible
1) Find the profile folder of the profile containing the Outlook mails (e.g. %AppData%\Thunderbird\Profiles on Windows).
2) Within this profile folder find the files that have the same name as the folders in Thunderbird. There should be one file with this name and one file the ending .msf
for example:
%AppData%\Thunderbird\Profiles\i2m1oyie.default\Mail\Local Folders\mail-dir
%AppData%\Thunderbird\Profiles\i2m1oyie.default\Mail\Local Folders\mail-dir.msf
copy the files without .msf
%AppData%\Thunderbird\Profiles\i2m1oyie.default\Mail\Local Folders\mail-dir
3) Within the other Thunderbird create folders with the same name (e.g. mail-dir)
4) Find this profile folder and replace the copy the files from the "Outlook-Thunderbird" here.
This will integrate your Outlook mails into your existing mail account.
Also see the Mozilla Knowledge Base articles about this topic:
http://kb.mozillazine.org/Importing_and_exporting_your_mail
http://kb.mozillazine.org/Importing_folders
http://kb.mozillazine.org/Import_from_Outlook_Express