Here is one idea for you to consider.
Add the same label to all the test cases you want grouped under your new User Story (example TC_NewUserStory#). You should be able to do a bulk update on the test cases, if they are already grouped together, to add the new label.
When all those test cases have the same label on them, do a search on that label name and save the search. (Go to ISSUES > SEARCH FOR ISSUES > do a search; "labels=TC_NewUserStory#". Then save that search.)
When you go to relate test cases to something, there should be an option for VIA SEARCH FILTER. Click that and type in the saved search you just did.
Now all your test cases should be present.
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