Yes, titles in global collections can be modified via KBART or individually in Collection Manager. For the most part, any record contributions/changes will be retained and not overwritten by providers' updates. However, there are a few exceptions: if the collection is an "OCLC Cataloged" collection, which is a label that you can see in the Type / Attributes column when viewing collections in Collection Manager, then OCN changes may not be retained and may revert back. Non-OCN changes will stick, including adding new titles to the collections.
If you would like to make OCN changes to an OCLC Cataloged collection, please contact OCLC Support with the collection ID and your requested changes.