You can add Header and/or Footer to your grid by using ng-template within your angular-slickgrid component, it's as simple as that. Using these slots also has the advantage of being contained in the same container making them the same width as the grid container.
<angular-slickgrid gridId="grid1" [columns]="columns" [options]="gridOptions" [dataset]="dataset">
<ng-template #slickgridHeader>
<h3>Grid with header and footer slot</h3>
</ng-template>
<ng-template #slickgridFooter>
<custom-footer></custom-footer>
</ng-template>
</angular-slickgrid>