You would read your HDL and elaborate as follows:
read_hdl -v2001|-sv <rtl_files>
read_hdl -netlist <ip_netlsit>
Then, you would preserve the IP block:
set_attr preserve true [find / -subdesign <ip_module_name>]
Continue with synthesis as you normally would. The IP module will not be modified, but the paths to and from the IP module will be optimized.