It is rather time consuming to create an impact assessment method than complicated.
1. You should create at first a new empty assessment method in openLCA - with title and despription etc.
2. Export your freshly created method and the method, which contains the specific category e.g. the ILCD method.
3. Extract the second - so to say already existing - method.
4. Do NOT extract your empty method, that is important, after extraction no import is possible (I dunno why...)
5.Open for both impact methods the folder: "lcia_methods"
6.Open the .json files with an editor and copy the content in "https://jsonformatter.org
". The formatter makes the file easier to read.
7. Now copy the name and all to the impact category belonging details in your new impact method. For sure only the categories you wanna have in your new method. Add this information in the .json file of your new method.
8. Now open the folder "lcia_categories" folder - find the categories you need and copy those into your new method.
9. Finally you just need to copy all folders, which you are havent touched yet into your new impact method (categories, unit_groups etc.)
10. Import your new method into openLCA - best test is in an empty database. Be sure there is no method existing with the same name or any folder which might be created due to the import process is existing. This could cause problems.
First try should be done with just one category.
I hope this workaround is helpful. It worked for me but it took me a bit.