Depends on the tool.
Some reporting engines provide web services to get report data; in this scenario you could invoke the api on the engine to get the data and then build a smart portlet UI to render it.
Other more limited tools only have web presentation. Here your options are to scrape to extract data to display yourself or leverage either the web proxy or iframe portlets to expose the tool's presentation. iframe is typically the selected method here because the UIs tend to be a little complicated for the web proxy portlet to deal with. The challenge here is the support for authentication; iframe won't pass through so users typically have to log in via the iframe.
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