If we're talking about most recent versions of uCMDB & NNM - there's a special Discovery Job which pulls out data from NNM (using web services) and populates uCMDB with it. AFAIR, there's a readme coming together with the NNM_Integration package which describes the how-to.
The package should work with BAC 7.5 as well, but your problem is not a BAC version. The problem is the NNM version as the out-the-box integration was developed based on new web services available in NNMi. As a result you need NNMi 8.10. The integration won't work with the old NNM 7.5 just because those web services don't exist in that version.
The only thing you can do is to develop your own discovery pattern for NNM 7.5, or maybe someone who did it can share it with you. Or consider upgrading both BAC and NNM to the versions with ready-to-use integration.
OMW/OMU integration with uCMDB is done through OM add-on called DMA (Dependency Mapping Automation). DMA 8.0 works with OMU 8.27 and higher, OMW 8.0 and uCMDB 7.0; DMA 8.10 works with OMU 8.27 and higher, OMW 8.10 and uCMDB 7.0/7.5.
I have the same need but I have also some question in addition. With BAC 8.0 and NNM 8.10 the uCMDB-NNM integration is out of the box. But can i see in a BAC view also the status of NNM nodes? How are nodes and elements mapped in the uCMDB? If i have in NNM a router with several interfaces, how are such elements reported? One CI for each element? Are the relationships mantained?