All depends on the system. If you are going to replace it probably the easiest to configure and set up would be a texecom system. You can buy complete packages and they also do a wireless system. They have probably the easiest set up manual.
Other alarm systems to look at would be Galaxy, Scantronix, castle and menvier.
If it is a newer alarm system then there shouldn’t be a reason why you cannot have multiple remote keypads. They would normally be addressed so the system knows what which one is doing.
It is possible that your current system may be limited due to age etc. with regards to running keypads in parallel again it would depend on the system.
I’d be inclined to give a local alarm firm a call. They may give you a free estimate which could well be cheaper and much less hassle than ripping the whole system out and replacing.
Good luck with it 🙂