If you have existing metadata records in a library catalog or finding aid, you can repurpose this information in batch for your digitized items for use in digital collections or other purposes. It will depend a bit on what systems you are using but the overall steps are likely the same.
There are methods to extract MARC records from a library catalog into a spreadsheet where you can then map the fields to the new schema. Many systems allow for metadata to be uploaded with your digital files via csv file made from your spreadsheet. You also can re-purpose finding aid description for archival collections.
If you have digitized items you would like to put into a CONTENTdm instance you can import them in batch to CONTENTdm using a csv of metadata and a folder of your image files. In your spreadsheet (csv), one row of metadata corresponds to an image file or files - (this setup does depend on system, this example is CONTENTdm)
Metadata from a library catalog
You can get your metadata into a csv file from your library catalog using MarcEdit.
Metadata from a finding aid
For finding aid metadata it sometimes can be as simple as copying and pasting box and folder information into a spreadsheet and duplicating it for each digital file, then batch adding in other metadata like Type, Format, Language. Scripts can be written to automate pulling metadata from XML formatted finding aids as well.
An example can be found here http://pfch.nyc/repurposing_archival_metadata/index.html
Once your metadata is exported into the csv you can rename the column headers with the correct CONTENTdm dublin core field names and batch clean up/normalize the metadata values using Excel and/or OpenRefine. You are then ready to import into CONTENTdm.
NOTE: Your csv or spreadsheet of metadata will need to have column headers that are the metadata element field names. Some systems require these to be in a specific order and/or named in a particular way. For example in CONTENTdm you can name your fields anything you want them to be and that name is what they need to be in your metadata, and in that order. For CONTENTdm the easiest way to get this right is to first have your metadata set up in CONTENTdm, then export as a tab delimited file to get the column headers. Instructions for how to do this are here https://www.oclc.org/support/services/contentdm/help/collection-admin-help/exporting-metadata/exporting-to-tab-delimted-text-files.en.html#par_text_b809