Not completely, but there are ways to achieve this partially. You can add multiple sources (say, stable and unstable) and set the priorities so that by default, stable is preferred. Then you can choose to install certain packages from unstable. However, since these frequently depend on newer libraries (e.g. libc6), it's hard to pull this off without upgrading most fundamental libraries to the version from unstable. In that case, you might just as well run unstable.