If your organization is a legal entity (a business or otherwise officially recognized organisation), then the copyright on the software that is developed on behalf of your organization will most likely belong to your organisation.
As the copyright holder, your organisation can change the copyright on their code whenever you like and that includes attaching a license to code that did not previously have an (explicit) copyright license.
When choosing a copyright license, there are a number of considerations to take into account:
- If you want to use another license than a well-known open-source license, you must involve a lawyer in the drafting of the license text and in verifying that your license is compatible with the licenses for third-party code that you depend on.
- For changes made to third-party libraries, you must check what requirements the license places on you for under which license you can release your modifications. The easiest option is to always release those changes under the same license as the original library.
- If your application makes use of of a GPL library, or a library under a similar (strong) copyleft license, then you are effectively also required to use a copyleft license.