I've got few NetFlow dumps captured by nfcapd deamon. Is there any possibility to convert them to .pcap format so I can analyse ones with my software?
Basically no; most of the information from the packets is lost, including the entire payloads. NetFlow summarizes the header information from all the packets in a given session: it could be a dozen or thousands. The NetFlow dumps do not (to my recollection) include partial updates either. So, you can go one way (convert from pcap to NetFlow) but not the other way.
That said, if all you need for your analysis are the IP headers of the first packets, you might be able to fake something. But I don't know of any tool that does it.