First, your question is not in right forum (though in right Forum Nokia). This discussion board is for MMS developers and it has not(hing/ too much) to do with MSC.
To your question in general level. To my knowledge there is no API available (as open source) for CDRs and CDR files. This is the same for MSC, MMSC, SMSC, NAP, NAWG etc. The principles for these are very much the same and I really don't know, why there is not that kind of API available. Keep asking and maybe someone will create them. (I just used to work with billing in my previous life and creating that kind of API is not that difficult, we made couple of them).
Please also remember, that Nokia MMSC support many kind of billing/charging scenarios including real-time In-Advance Credit Check (IACC), where the API is a different one.