There are 2 options to add Retargeting/Remarketing traffic back into your primary funnel. Often, Remarketing traffic will convert at a better rate than your regular (cold) traffic. Also, Remarketing traffic costs are usually different than the initial traffic cost. You can define all this when adding a Retargeting Traffic Source (see Entry 2 and Entry 3 below).
This image below shows you some ideas about how to incorporate Remarketing traffic back into your main funnel:
Another option is to automatically add Retargeting to the N-path traffic of a funnel object as shown below...
Step 1: Attach a Retargeting Entry to your N-path
Step 2: Set up the required metrics
The CPC (Cost Per Click) is usually more expensive than the CPC in your initial ad. You can adjust your CPC values accordingly.
Ratio % - If you only added one Traffic Source to your Retargeting Entry, leave this at 100%
If you added two Traffic Sources to your Retargeting Entry, you can set each at 50%. This would distribute 50% of the Retargeting traffic to each of the traffic source's Retargeting ads.
Revisit Rate % - Only a certain % of your "lost" or No-path visitors will revisit your webpage after seeing a Retargeting ad. Some industry reports show that on average, 26% of visitors will return back to your website after seeing a Retargeting ad. (source: https://www.invespcro.com/blog/ad-retargeting-2/)
You can use that 26% as a general guideline as shown below...