It is much nicer to have an index that adapts itself to all changes but the adding/deleting/rearranging of \index commands. Then one only needs to run makeindex after such a change. The third party can add its pages without causing trouble.
Our package robustindex.sty (version 2019/01/25) achieves this with the \pageref mechanism.
Here is our Manual for the robustindex package.
Just try what happens if you put \usepackage{robustindex} in your preamble. Do read your TeX log file.
The sample file robustsample.tex illustrates more than you ever need. It also requires the package robustglossary.sty.
After the old fashioned
If you want several indexes with pagerefs you may try \usepackage[multind]{robustindex}.
After