Plugboard destination field series_index

[principle]

Is there any way to update the series index via the plugboard I see no destination field for it?

So I have series sorted into series and subseries, (subseries is a text column for keeping series like information)

if i do this it works fine {series:'ifempty(field('#a'),'field('#b')' )'} however it's still seems to be reading the series index from the series column so even if column #b is set to 'book[15]' the metadata for series_index is still [1]

Is it possible for me to add a series index destination field?

Thanks for taking a look