How to link requirements using Custom ID rather than Index

We have requirements in a third party tool that we want to link to a model but we still need to manage them in the third party tool. We import the requirements into the Requirements Editor by using reqif import. When we re-import the requirements, due to a change that removes requirements, adds requirements (other than at the end) or otherwise re-orders requirements, then the existing link information cannot be re-used. How can we have the link information be maintained on the basis of the CustomID rather than on the ID?