If I understand correctly (I'm not too technically savvy): there are many ways I can create a keyword listin Bridge using the existing metadata -- but then I would have to manually reenter the keywords from the list into each individual image?
I have a lot of image files. Can I somehow convert the IPTC keywords in each file into Bridge keywords?
No, I think you misunderstood.
If the old/other software wrote the keyword metadata to the files using either the legacy IPTC or the modern XMP-dc methods, then Bridge should recognise that there are keywords in the files.
The Keywords panel in Bridge is for storing and re-using keywords to apply to other images. If you wish to transfer the keywords from your files to the Keywords panel, there are a couple of methods:
1) Export the keywords from your files to a text file and then import the text file so that the Keywords panel has a persistent list of entries, that you can put into groups/categories.
2) Browse your images so that the keywords in the files that do not exist in the Keywords panel show up in italics under the "other keywords" category, then right-click each of them and make them "persistent", that you can put into groups/categories.
https://helpx.adobe.com/au/bridge/using/keywords-adobe-bridge.html
Either way, you need to decide if you are going to use a simple "flat" keyword structure, a Parent/Child keyword structure or a Hierarchical keyword structure.
https://helpx.adobe.com/bridge/kb/nested-hierarchal-keywording-bridge.html