JIRA supports a wide range of imports via the JIRA Importers Plugin available at https://plugins.atlassian.com/plugin/details/32645?versionId=773420. It doesn't support MKS directly, as far as I know, but if you can export to any of the following formats, then you should be able to successfully import your issues.
If you don't have very sophisticated requirements wrt the shape of data (like issue links, attachment history, user mapping, etc.) - I would go with CSV import - fully configurable from web UI.
Alternatively you could play with basing on a jelly script or, if can afford a little bit more effort (but get the best results), you can always extend JIRA Importers Plugin on your own, adding the support for MKS. This short info may help you doing that.
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