[Koha] Import items (holdings)

My current system exports the MARCs and the holdings into separate files. The MARC export routine inserts a item unique number into the 910 field, and the Item export includes this unique number in the item export file. I've imported my MARCs into Koha just fine, but I cannot figure out how to bulk import my holdings and then have them link properly to the MARCs. Can anyone help? Thank you!