[Koha] MARC Import

Hi all! We are currently looking into importing our ISIS dataset into koha. However, the ISIS database was designed using non-standard conventions fo the MARC21 fields, so we need a "conversion" first. I found out that Koha supports exporting the data into custom MARC formats, ist this also supported for importing (so e.g. can I create a definition of my format and import then directly into Koha)? Thanks for any answers, Wolfgang