You can't just yet (Fall 2017), but this is one of the many things that we are looking at.
For now, however, there is a quick workaround: you can copy an existing message with one click to create a new one from it.
So you can sort of using existing messages as templates.
Is there any way to share a bee template between different accounts?
In BEE Pro you can now move a message to different Project, but you cannot move it to a different account. With regard to template management, we are looking at introducing some new functionality in next releases. If you have specific product feedback on that, please let us know.