You can include any content inside your snap, including other snaps. In some cases snapcraft may not have the right helpers in place to do what you want with a couple of lines just yet, but you can always create a makefile that does exactly what you need to be done at build and install stages, for example. Use unsquashfs to extract the previous content from the other snap.
That said, consider if the first snap is supposed to be consumed on its own. If it's not, having that snap lingering in the store under your account won't be great when nobody can actually install it in a useful way. We need to improve snapcraft to handle reuse better in these cases, but meanwhile using a deb would be fine if you're already comfortable with that, or even simply pushing the built piece to a place you can share across consumers for the time being.