The question will be will you have access to the RTL code to implement it in the PL? I do not think ARM provide this. The IP you see in the catalog is only to configure the Processing System (PS) which is an hard IP. This is why you can only had one in vivado, because you only have 1 PS.
However, you have 2 solutions in the PL if you need more processors:
Use the microblaze processor which is the Xilinx soft processor
You could implement an ARM cortex-M1 or ARM cortex-M3 (refer to ARM website for details)
Hope that helps,
Florent Product Application Engineer - Xilinx Technical Support EMEA **~ Don't forget to reply, give kudos, and accept as solution.~**