The only way to do this currently is to use a code solution. You can override the footer by hiding the standard flow footer and adding a custom component to the bottom of your flow screen.
Here's an example from the documentation (link below):
<aura:component access="global" implements="lightning:availableForFlowScreens">
<!-- Determine which actions are available -->
<aura:attribute name="canPause" type="Boolean" />
<aura:attribute name="canBack" type="Boolean" />
<aura:attribute name="canNext" type="Boolean" />
<aura:attribute name="canFinish" type="Boolean" />
<aura:handler name="init" value="{!this}" action="{!c.init}" />
<div aura:id="actionButtonBar" class="slds-clearfix slds-p-top_medium">
<!-- If Previous is available, display to the left -->
<div class="slds-float_left">
<aura:if isTrue="{!v.canBack}">
<lightning:button aura:id="BACK" label="Previous"
variant="neutral" onclick="{!c.onButtonPressed}" />
</aura:if>
</div>
<div class="slds-float_right">
<!-- If Pause, Next, or Finish are available, display to the right -->
<aura:if isTrue="{!v.canPause}">
<lightning:button aura:id="PAUSE" label="Pause"
variant="neutral" onclick="{!c.onButtonPressed}" />
</aura:if>
<aura:if isTrue="{!v.canNext}">
<lightning:button aura:id="NEXT" label="Next"
variant="brand" onclick="{!c.onButtonPressed}" />
</aura:if>
<aura:if isTrue="{!v.canFinish}">
<lightning:button aura:id="FINISH" label="Submit"
variant="brand" onclick="{!c.onButtonPressed}" />
</aura:if>
</div>
</div>
</aura:component>
One limitation of this is that it won't automatically fix the buttons to the foot of the screen in quick actions, since flow doesn't understand that your component is a footer and not just a generic screen component.
See:
