Scoped slots give you greater control over how the record data appears. If you want to add an extra field which does not exist in the records, just add it to the fields array, And then reference the field(s) in the scoped slot(s).
<b-table
no-local-sorting hover show-empty
select-mode="multi"
selectedVariant="success"
:items="this.products.items"
:fields="this.products.fields"
@row-selected="rowSelected"
@filtered="filtered"
>
<template slot="mybutton" slot-scope="data">
<b-button variant="outline-primary">Edit</b-button>
</template>
<template slot="empty" slot-scope="scope">
<div class="text-center">
<h5>Пусто</h5>
</div>
</template>
</b-table>