You'll have to create a new BUY button at Paypal side, and then get the link (or the code, depending on which you prefer best). Afterward, whenever a user clicks your button within Clickfunnels instead of taking them to the internal order form step, take them to the link Paypal gave you.
You'll have to add a return URL to your Paypal configuration, to take your clients once the finish purchasing back to the next funnel step to (maybe) upsell them more.
Please notice: Using Paypal is way not recommended, especially when having Upsells/Downsells in your funnel. Testing has proven that Paypal can really damage your conversion rates.