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Insert > Web Component > Included Content > Page
That will bring up a dialog which allows you to browse to the page you wish to include. Select it, then press okay and you should see that new included content appear on the page. In the html view, you will see something like this...
<!--webbot bot="include" u-include="/file.htm" tag="body" -->
That is FP's code to execute its interpretation of an SSI or Server Side Include.
Usually I don't have the type of situation you have painted. If content needs to be included, usually it occupies a scope of pages and therefore the standard FP includes work just fine. If that content appears on more than two pages, it may be of benefit to utilize the FP include feature. If not, just cut and paste (the old fashioned way).