The short answer is no.
Options include:
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writing an external script that works with OH’s SSE and REST API to translate all the Items to MQTT and the Homie convention.
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writing Rules in openHAB to do the same (see MQTT 2.5 Event Bus for an example of something similar).
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writing a binding
Notice though the amount of work is completely independent of the number of Items you have. You either use Group membership, Item tagging, Item metadata, or use the API to get all of the Items. The amount of work you need to do is the same for 1 Item and for 1000 Items.