With RO Payments it is not possible to use external party plugins. To make paymob either they need to develop a plugin like they did for VirtueMart or someone needs to develop an addon for RO Payments.
We do not do custom development, we cannot help you with a paymob plugin.
If you are not attached to paymob you could consider switching to or ysing a payment provider we do support. That way you can use the Membership Pro integration we have.
Hope that helps.