Dear Iohi21,
I added the pads after synthesizing my design in the verilog code generated by the rtl compiler.
You can find the type of pad as per your requirement from the library documentation.
In the new veilog file
Just add a new module eg. design_pads and instansiate the pads as u would instantiate any othe module in verilog.
When u are importing the design for placement and routing you have to include the appropriate led files and timing files which are there in the design library.