I'm afraid is not possible, or at very least, do not worth it. You can do the following:

• Set up a chrooted environment with the newer g++, coming from the repositories.
• Advantages: your installation is not touched. It can aslo be automated.
• Disadvantages: correctly setting up a chrooted environment sometimes is not trivial. Time.
• Building the packages yourself (backporting)
• Advantages: high personalization. No need to waste hardisk space
• Disadvantages: You need to build it again each time, so it's not automatic. The probabilities that your environment gets broken is higher.
• Using a virtual machine:
• Advantages/disadvantages: same as the chrooted solution, just that both systems are completely separated from the kernel.

I'm afraid is not possible, or at very least, do not worth it. You can do the following:

• Set up a chrooted environment with the newer g++, coming from the repositories. debootstrap should help you with this.
• Advantages: your installation is not touched. It can aslo be automated.
• Disadvantages: correctly setting up a chrooted environment sometimes is not trivial. Time.
• Building the packages yourself (backporting)
• Advantages: high personalization. No need to waste hardisk space
• Disadvantages: You need to build it again each time, so it's not automatic. The probabilities that your environment gets broken is higher.
• Using a virtual machine:
• Advantages/disadvantages: same as the chrooted solution, just that both systems are completely separated from the kernel.