I have my laptop which got Windows 7 64-bit OS. For development purposes I have installed Windows 7 32bit on VirtualBox on same laptop.
Is it possible to use same key for 32-bit OS on VirtualBox while having 64bit Windows on laptop?
Legally no. You are breaking the EULA.
You can use the same key on 32 or 64 bit Windows of the same edition (e.g. Home Premium, Professional etc.). But, unless you have MSDN or similar, you are only allowed one installation.
The answer is no, and you can't since: