With traditional symmetric encryption, this is impossible – you need to encrypt a file with exactly the same key it'll be decrypted with.
What you're looking for is asymmetric, or "public-key based" cryptography, as used in PGP and SSL. In asymmetric algorithms, you have a pair of keys, one for encryption (public), other for decryption (private). The encryption key can be made publicly available and anyone could use it. (It's how HTTPS works, by the way.)
Unfortunately, I'm not aware of any "archive"/"container"-type programs that work this way, mostly because it's somewhat tricky to create append-only archives reliably – either you have to store the metadata unencrypted, or you run into serious data corruption risks. However, it works well if you only have one output file per input, such as...
...One possibility is to install portable GnuPG, a free PGP implementation, and write a script to automatically encrypt files into a "temporary" folder. Later at a secure location, you could decrypt these files with GnuPG again and move them into your TrueCrypt container.
For example, this
.cmd script would encrypt all files drag-and-dropped on it:
@echo off & setlocal
:: I assumed a USB stick, so here %~d0 will expand to the drive letter.
:: Where this script looks for GnuPG
:: Where GnuPG looks for its keyrings
:: Where the encrypted files are put
:: Your PGP key ID or email
if not exist "%DESTDIR%" mkdir "%DESTDIR%"
if "%~1"=="" goto :eof
echo Encrypting "%~1"
"%GNUPGDIR%\gpg" -r "%RECIPIENT%" -e -o "%DESTDIR%\%~nx1.gpg" "%~1"