How can I get the data from PagingData<T> object before it is reached to ViewHolder?

Question

I want to get the data of a PagingData<T> object in some in-between class like ViewModel before it is reached to the Adapter and gather its property as a list. I can access the code by the flatmap but I don't want to apply any changes. Also, this kind of access is useless because the entire block runs after subscribing to the observable is finished in ViewModel so it is unreachable by the ViewModel running.

    private fun getAssignedDbList(): Flowable<PagingData<T>> {
        var list  = mutableListOf<Int>()
        return repository.getList()
            .subscribeOn(Schedulers.io())
            .observeOn(AndroidSchedulers.mainThread())
            .flatMap { pagingData ->
                Flowable.just(pagingData.map
                { foo -> 
                    list.add(foo.id)
                    foo })
            }
    }

Another example, let's assume that we have this scenario: I'm going to call another API before the Paging data is reached to the view holder. The paging data needs to be filled with a new API call response. How can I wait for the second API call to update the data of paging data?

 private fun getListFromAPI(): Flowable<PagingData<T>> {
        var list  = mutableListOf<Int>()
        return repository.getList()
            .subscribeOn(Schedulers.io())
            .observeOn(AndroidSchedulers.mainThread())
            .flatMap { pagingData ->
              //call another API calls and waiting for a response to change some of the fields of 
              //paging data
            }
    }