Show communities spinner (in Napili) via custom lightning component

You can create a component for the spinner and render only when you want, with something like a flag or even a event and hanlde with js.

<aura:attribute name="toggleSpinner" type="boolean" default="true"/>

<aura:if isTrue="{!v.toggleSpinner}">
      <c:mySpinner/>
</aura:if>

You can check this section of LDS if you want more info

Spinners