I would say not really. Even if the Backup tool has not changed and any structures (within the 800xA code) has not changed, you would find you'd have many, many errors and/or make a havoc of the 5.1 system once imported.
It is highly recommended not to do this. It would be better to upgrade the 5.1 system to the same version you are at. This would give you a level playing field (ie same versions) and therefore a higher chance of success.
I fully agree with Ingo. Was your application developed in Control Builder M ? If you really have no choice but to downgrade CBM logic to V5.1 , there is one way that requires a lot of efforts. CB Open interface contains functions that can be used to export Programs, CMs, SCMs , FBs as xml text source code. Next step is to cut all new features of V6 using any text editor. Finally, you need to import modified code back to CBM V5.1 using CBOI scripting. Only one way how to find all V6 properties is try and error. I went through this suffering once successfully but it took about 2 days. if you want to do the same contact me by email off forum, I'll guide you.