I think the short answer to the question in the title is "No".
See the iTunesConnect > FAQs > Manage your applications
I sold my app to another developer and
can no longer distribute on the App
Store. Can I transfer the app to the
new developer's iTunes Connect
account? At this time, applications
cannot be transferred to another
developer account. If you would like
the application to be sold through
another developer account, you will
need to remove the app from sale in
the current iTunes Connect account and
upload the app under the new iTunes
Connect account.
Not exactly your question, but sounds similar enough.
We had a need to resign a 3rd party developer supplied app with our signing keys - which sounds similar to what you want to do:
1) Use Xcode's Build > Build and Archive feature to create the version of the app to had off to your customer.
2) Select that build in the Archived Applications source in the Organizer
3) Choose Reveal Archived Application in Finder from the contextual menu
4) Select and compress the enclosing folder
5) Send that zip archive to your customer.
Your customer would then unzip and expand that archive in ~/Library/MobileDevice/Archived Applications. That will make it available in the Xcode Organizer. Your customer can then use the Share Application… and/or Submit to iTunes Connect… buttons to re-sign and distribute the app using your customer's distribution provisioning profile.
EDIT: A follow up that may not be clear from the info above.
To be able to do this, the 3rd party developer needs to build the app with your development profile; e.g. they need to be a "Team Member" of your account. They can do nothing more than build the app and sign it with development keys. Then they can deliver a build that has been compiled with your development keys at which point you can re-sign this with your distribution keys and submit.
If you need to, you can remove them and revoke their information from the iOS Provisioning Portal after successful app deployment.