Excel: add new members to existing methods / functions using the VB editor

Question

So I have this huge, messy dataset which is supposed to show various tools and a record of when they're H&S checked. The problem is there's an awful lot of instances like this:

as you can see, one tool has multiple rows, but could easily be concatenated down to a single row. I don't want to do this by hand so I tried making a macro using the subtotal feature, then going into the macro editor and replacing the xlsum function with xlconcat:

which sprung an error:

So i followed the range class and found the subtotal method and saw the applicable members were taken from xlconsolidatefunction. Perhaps this is a daft and overcomplicated way of doing what I want to do, but is there a way i can add the concat function into the memberlist of xlconsolidate function?