From looking at their Github, they have a PHP SDK so probably wouldn't be too tricky.
Note that we only integrated Stripe to do one-off payments, rather than recurring. Invoices are recurring but you have to manually pay them each time.
Again probably worth mentioning neither myself or Johnny are "developers", we're just sysadmins who write code.
My only reservation about adding more payment providers is that it's a little like the whole "integrate ITFlow with X" discussions we get - ideally we'd like people to use the API - but I get for payments writing your own API integrations is probably quite scary and dangerous.
But when does it stop? Should we add all the big players too (PayPal, BrainTree, Amazon, etc)?