It's not an Atlassian add-on really, it's by Spartez. I'd actually re-ask this question, but with a tag of addon-com.spartez.jira.plugins.bork.tfs4jira as I think Spartez monitor that. Also, it might be worth raising the exact request on their issue tracker.
Can you be more specific about this custom field in question? Generally, we do support synchronization of wide range custom field types. But Nic is right. Best way is just to contact us at firstname.lastname@example.org and we will try to answer all of your questions and seek for possible resolution.
[Spartez Support Team]
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