I want to use PmodIOXP to allow both PmodAQS and PmodHYGRO to be connected into the same port in Zybo Z720. However, I do not know how to write my block design in Vivado since there is no IP for PmodIOXP in the vivado-library. Also, what kind of C++ code should I write in my Vitis after I exported my vivado file into Vitis
PMOD IP is not provided by Xilinx. It's a 3rd party IP provided by the vendor who provided the module.
Usually, vendor provides a repository in a package that we need to point in Vivado for IP to appear in IP catalogue. I guess Digilent provides library for this IP. Please have a look at following page. It also provides steps to install this IP in Vivado.