[Koha] Converting ms-access database to Koha

I am a non-specialist librarian now running a library of around 500 books. I decided to try out koha and have successfully installed the windows version. Since the library computer does not have an internet connection (due to LAN security policy), i am presently entering the isbn in bookwhere and then taking the marc records on a usb stick and uploading them into the koha reservoir. Since our many of our books are quite old, i am getting a hit rate of just 20 -30%. We already have an ms-access database of the complete collection with accession no, author, book name, price, etc. Is there any way that i can directly upload this data onto Koha (if possible, only the ones for which i dont get marc data). Thanks in advance, Nitin