Our system has no way of handling recurring payments such as a periodic subscription or installment plan; however, you can use PayPal Recurring Payments with PayPal's own Subscription buttons at the same time as using E-junkie with PayPal, and they won't interfere with each other at all.
You may wish to combine that with a purpose-built membership management system such as FrontDeskApp, CheddarGetter, MemberWing, etc. If you set up IPN manually in your PayPal account settings for any recurring payments solution, E-junkie's buttons will override those IPN settings for orders placed through our system without affecting IPN for Recurring Payments.
The main problem with supporting such a feature at our end is that, even if we could send orders to checkout that would initiate a recurring payment plan, we have absolutely nothing built into our e-commerce system that could provide buyers and sellers with anything of value for those periodic payments, such as tracking membership/subscriber status, administering access to a members-only system, scheduling mailings/downloads, etc.
While we do use PayPal Recurring Payments to manage subscriptions for the use of our own services, that isn't integrated with our cart or sales-processing system at all. We just use PayPal's standard subscription links to help our sellers set up a payment schedule in their own PayPal account, telling PayPal to send us the same amount every month, so we just passively receive those payments to mark the seller's account as active for another month.