Mashery has some mechanisms for downloading endpoints from there platform that then can be transformed into proxies in edge. In the past doing this sort of migration i typically create a pattern or a template proxy based on the logical variations the existing APIM solution presents and then use a script insert the endpoints into the correct template.
A more manual approach would pay off more in the future though because it will give you a chance to clean up any technical debt that you may have accumulated over the years on your existing platform.
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