Hi @Mcintosh, Rebecca ,
Per my research, I found a similar issue. You can refer to the following links to create ddf file to repack wsp.
https://learn.microsoft.com/en-us/previous-versions/office/developer/sharepoint-services/bb466225(v=office.12)?redirectedfrom=MSDN
https://sharepoint.stackexchange.com/questions/31574/making-a-cab-file-with-ddf-to-create-a-wsp-solution-package
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Yes managed to get it to work by using SharePoint WSP Repackage from GitHub.
Hi @Mcintosh, Rebecca ,
Glad that you found the solution to this problem. Thanks for your sharing.